Andy Warhol, Albert Einstein and my boy Tom

It doesn't take an educational psychologist to tell a mother that her child is special. I knew already that my child was special, but it was a different kind of special then. I knew he was extremely bright and articulate, yet I saw him struggle with the subtleties of social interaction, missing social cues and shunning peers. I knew that he could read and write before any of his friends in nursery school, but also that he wouldn't play "house" or "chase" with them.

As parents, we dismissed Tom's clumsy attempts to catch and throw a ball, and focused instead on his promising studious nature. At four years old, his voracious appetite for facts and figures, combined with self-imposed rituals - such as writing down every street name when we went on trips - didn't seem odd to us. His total recall of rote-learned facts from books and maps just seemed to confirm his standing as an eccentric genius in the making.

My then-husband and I joked about how we thought we might be raising anything from an anorak to a world leader. It never occurred to us that there might be something wrong.

A drawn-out separation and divorce from Tom's father was followed, some time later, by a move from one end of the country to another. Tom didn't settle at school right away. Despite (reluctantly) having classmates around to tea and joining a drama group, he seemed, somehow, very detached from everything external to home. His teacher and I arranged to talk about his progress.

The school doctor and an educational psychologist suspected Asperger's syndrome, which affects up to 25 children per 10,000, mostly boys. Many people go through their life undiagnosed, labelled as (and believing themselves to be) eccentric, socially inadequate or clumsy.

The diagnostic criteria for AS talks about qualitative impairment in social interaction, restricted patterns of behaviour, limited interests, speech and language peculiarities and repetitive, self-imposed rituals. One researcher, Uta Frith, has characterised children with the disorder as having "a dash of autism".

The news didn't register properly until later that evening, when I typed the word Asperger's into a search engine on the Internet, as Tom sat on the floor constructing a complex obstacle course for his home-made robots. I printed off everything I could find on the subject, then spent the rest of the evening reading description after description of my son.

It's an uncanny experience to read all about your child's personality when, just hours earlier, you had occupied a world in which you celebrated how individual and different he was. Then comes the realisation that the child's academic excellence and eccentric behaviour form part of the differences that make him the same as many others, and that those differences may eventually be a reason for others to discriminate against him.

The school doctor devoted an hour and a half of her time to discuss and observe Tom. His entire history was potted into that 90 minutes. Had I had a happy pregnancy? What was the birth like? In fact, the often-told story of Tom's horrendous birth reads like a roller-coaster of horror and hilarity. Two naive newlyweds, armed with a bagful of essential oils and a birth plan that shunned chemical pain relief, were no match for the 15 hours of sheer agony that was to come.

One study stated that nearly half of the recorded cases of Asperger's syndrome had a history of pregnancy-related conditions that could cause brain damage, a significant proportion of which recorded obstetric crises in the latter stages of labour. Tom's forceps delivery would probably fall into that category.

And on she went. Had we noticed anything odd about his social skills when he was a toddler? At what age did he walk and talk? Tick, scribble, tick, tick. I started to feel stupid for not having realised sooner that something was wrong. By the time Tom was invited into the room, I felt there was little else to say. As always, Tom rose to the occasion with a flourish.

The doctor gave Tom a piece of paper with four shapes drawn on it and asked him to "draw the same shapes underneath". He obligingly picked up the paper to consider whether to draw underneath the sheet on the desk or on the reverse of the paper (literal use of language is a common feature of Asperger's). When, later, the doctor asked Tom to sign a picture of our new house that she'd asked him to draw, he asked her whether he should write his name, or "Boy of Destiny". When she later asked me, in private, what he had meant, I explained that it was a catchline he was currently using for himself.

The doctor really engaged Tom when she asked him whether he had been to see the school nurse recently to check his eyesight, height and weight. Here was an opportunity to recite statistics - to test himself, not to show off. "Yes. I stood six metres away from a chart and read all the letters. She said my eyesight was perfect." Weight? "26 kilos." Height? "123cm." The doctor looked in the class records. He was, of course, absolutely spot on. Had he had his eye test in this room? "No. This room's not big enough to stand six metres away from an eye chart." Well spotted, darling.

When I collected him from school later in the day, he made no mention of the doctor coming to the school to see us. Instead, he recounted the ground clearance figures of robots featured in the BBC2 programme, Robot Wars, as if he were speaking to a captivated audience, rather than to his mum negotiating the traffic between school and home. Later on that evening, Tom ran into the kitchen with an urgent message: "There's a Robot Wars special on Friday! We need to tape it!" Of course we do. How else would he be able to replay and scrutinise it for weeks?

The doctor said that she'd be in touch again soon.

After some research, I found that it would take until at least the next school year before Tom could be diagnosed within the education system. I contacted a private child psychologist and arranged for her to see Tom, dashing in the process another long-held principle. The diagnosis would cost pounds 60 per hour, and could span more than five hours in all. At about the same time, as it happened, my main source of income was typing an e-mail to let me know that he wouldn't be needing my services for the next couple of months.

The psychologist needed to spend an hour with me before she saw Tom. The questions were similar to the school doctor's. I didn't know whether she saw me glancing nervously at the clock once the hour had passed, or whether she would make anything of it. She had barely looked up from writing what seemed like reams of pages. She stayed for almost two hours: pounds 120 and counting.

I had told Tom that a woman would be coming to our house to see him and talk about how he was settling in to his new school and surroundings. He took the information in his stride and looked forward to her arrival.

When she came, she took him into the kitchen to have a chat at the table and go through some tests with him. They adjourned to his bedroom after an hour or so, and the bored little boy who had slumped in the kitchen became animated as he described the mazes and obstacle courses for his robots.

The psychologist came downstairs with the familiar glazed expression that all our visitors get after a while. She'd be in touch next week.

It wasn't a good time to take the phone call. I always get to the school gates at least 20 minutes before Tom comes out of school. That way, he can immediately spot the car and won't get distressed trying to find me. Ten minutes before I was due to leave, the psychologist was on the phone.

I had imagined a bold opening statement: Tom has Asperger's/ Tom doesn't have Asperger's. What I got was a list of things I already knew. Tom is very precise. He prefers to play alone. He speaks in a formal, adult way. His initial responses to questions are appropriate but then he goes off at a tangent. Does he have it? Does he have it? I need to get to the school gates - does he have it?

Yes. A positive diagnosis. Did I have any questions? A million - but I had to get to the school gates - and they're the type that come as I tuck him in at night, or as I check on him when I go to bed, or as I watch him in the school play, or as I iron his school uniform. My responses won't be neat and appropriate. Like Tom's, they'll be erratic; they'll creep up on me. They started at the school gates. Huge, hot, slow tears. A hundred questions, but no handkerchief.

The main question is the one nobody can answer. Now what? Children who have Asperger's today will provide some answers for tomorrow, but since there are yet no studies of the long-term outcomes for children with the syndrome, there are no statistics to draw comfort from or battle against.

In my search for information on the subject of Tom's potential future, I am terrified by the number of times the word "vulnerable" appears, but heartened by the shining examples of people who some researchers believe may have had this syndrome - Warhol and Einstein, for starters.

Images of Tom as a much-loved eccentric adult are juxtaposed with those of a shunned loner, because there is no room for subtleties or middle ground in my imagination. I reason that maybe those spotty, lank-haired teenage years when even the most socially adept of his peers will default into geekhood may help him finally blend in. Maybe his impossibly pretty face will help smooth his path. Maybe it'll increase his vulnerability.

After a weekend of alternating between trying to teach Tom to catch a ball, and crying in a soggy heap behind the locked bathroom door, I find myself fired into action. Today, I enrolled him into the school's excellent after-school club for just one day a week, starting on Wednesday. Judging by the ebullient children I met there, he's about to embark on a crash course in social skills. By Wednesday evening I'll know whether it's going to be the first of many, or just a clumsily clutched straw.