WHEN he heard there was a nail bomber in London last year he rolled on the floor, clutching his ribs and laughing loudly.

“Nail bombs!” He scoffed. “Why would anyone put NAILS in a bomb?”

We tried to explain that it was not amusing, that nails could be put in a bomb to maximise potential damage, but he was not having it. He found it hysterical and could not stop laughing.

Days later, he ran to me, white-faced and distressed. He was struggling to hold back tears.

“You know those nail bombs? I wish I hadn’t have laughed now. Because someone’s been killed.”

Whether he thought someone had been killed because he had laughed is unclear, but it took a certain maturity to admit that he had changed his viewpoint so dramatically.

A common perception is that people with autism live in a world of their own, which might have been true when my son was under five, but not now that he is aged 13. It is more like a difficulty in decoding certain situations. There is an understanding, but it is of a different kind.

He can impersonate Robocop with a physical fluidity of Michael Jackson doing the moonwalk, yet he has to concentrate on walking down a flight of stairs. I always know if it is him creeping down early in the morning. He only ever puts his left foot down first. The sound of his descent has a Christopher Robin-like bumb, bump, bump, to it.

He is what you call a high-functioning autistic. Some have preferred to say he has Asperger’s Syndrome, and some say Dyspraxia. He had much in common with a group of five or six children who were diagnosed at the clinic at the same time, which made me slightly suspicious that so many fitted exactly the same criteria. In IQ tests, there were some aspects, such as range of vocabulary, which put them in the top 10% of the population. In others, they fell into the bottom 2%. The tests they failed measured their ability to interpret situations. For example, a burglar drops a glove. A policeman sees him drop it and taps him on the shoulder to return the glove. The burglar gives himself up. The youngsters were asked: Why did the burglar give himself up?

Jake’s reply: “Well, there’s no point in him trying to run away because the policeman could fetch reinforcements and some of his colleagues could have guns or even tanks and things like that, and he wouldn’t be able to hide, because some of the police helicopters have heat-seeking equipment and what that does is to show up the temperature of your body from high up, and you wouldn’t be able to outrun a helicopter really.”

All of which reveals a certain amount of knowledge, but missed the original point.

Perhaps Jake just skips on too fast. Perhaps he considered the aspect of the burglar mistakenly believing he had been caught to be so elementary it was not worth mentioning.

His over-long explanations is one of his biggest difficulties in communication. He is unable to tell when the listener is becoming bored. It is hard for him to make friends. Or it might be that it is hard for him to accept he has friends, which is a slightly different thing. Unfortunately, in Jake’s mind, you’re either for him or against him. The slightest criticism - or a remark that he interprets as criticism - and he curls up like a sulky gnome, the bottom lip becomes prominent, and he mutters: “You hate me.”

You might expect that Jake’s difficulty in interpreting situations would lead to him having no sense of humour. Not so. One of his earliest jokes was one that he performed repeatedly before learning to walk. We had polished wooden stairs that he would slip down on his belly, feet first. He went at such a pace as to make people think he was falling down, and this was his party piece. As people ran forward to his aid, visibly startled, he would let out a long, wheezy, Santa Claus kind of chuckle. He was quite a rascal really.

All through his primary school years, he had a reflex action. If ever he thought someone had cracked a joke he would laugh loudly. It worked a charm on the teachers who were flattered that anyone should laugh at their cheesy jokes. I said as much to him one day.

“That’s what I like about you, Jake. You always laugh at my jokes, no matter how pathetic they are.”

“Was that joke pathetic then?” He responded, his eyes rounded in surprise. “I thought it was quite funny, actually.”

But he doesn’t just laugh at cheesy jokes. In the film Shooting Fish, he picked up on a background announcement which said the musical “Dogs” would be starting shortly. He was the only one in our family who immediately realised it was a play on “Cats,” and if he hadn’t have laughed, the joke would have passed me by.

His humour is by far the most charming aspect of his personality. If ever he is down, it is easy to cheer him up with the right triggers. His moods remind me of a lighthouse. You soon know his sunny smile can be made to beam again.

Other quirks in his nature intrigue me. He is protective of young children. He is a particular favourite of his cousin, who is four, and plays with her constantly. Recently he met a three-year-old for the first time and was instantly able to play with him, in a manner consistent with three year olds - a feat not many 13-year-olds would manage.

Jake would offer his seat on a crowded bus to a child under five, but not to a heavily pregnant woman. He recently told me, unprompted, that Jamie Bulger’s killers should not be released yet because, he said, he knew when he was ten that you can’t go around killing little kids.

It will always be difficult for him to take on board that he might have to modify aspects of his behaviour. Despite regular workshops at the clinic, which he decided to attend to boost his confidence, he will not talk about Asperger’s Syndrome and has relegated the topic to “Oh, that.”

I don’t try to broach it with him now, because he isn’t interested, but soon after the diagnosis, I was trying to explain what the psychologists had said. I picked as an example his ability to have a one-to-one discussion, but not a group one.

“Do you find it hard to follow what’s going on if a group of people are talking?”

He agreed that he did.

“Well, there’s a name for that, Jake.”

He just looked directly back at me and, after a moment, suggested: “Boredom?”