Notnagging, you are in a very similar position to where I was. I decided to tell DS2 before he went to secondary school, as once there he would be mixing with DC who had never met him before and I guess I wanted to make sure that he heard he had autism from me rather than from them.
I could also make sure that he heard about it in a positive way, that his first knowledge of ASD was that there were as many if not more positives to it as negatives. I didn't want him to feel it was an insult if he heard others using the term 'autistic.'
I kept it at his level of understanding and didn't use a book, because they all seemed to be aimed at DC with Aspergers and DS2 has HFA but not Aspergers. I told him that his brain worked differently to most of his friends, it meant he was good at Maths, remembering facts, computers etc (whatever applies, be over enthusiastic
) like Einstein, Bill Gates etc, but meant he found some things harder, waiting, thinking up ideas in literacy, friendships. (Again, whatever applies, especially those things that may be worrying him.)
Told him it was because he was on the autism spectrum (didn't mention disorder) he was autistic, like lots of other people. He might find some things more difficult at school, which was why Mrs. Xxxx worked with him sometimes, but that he was just as clever (hmmm) as anyone else.
He was completely unbothered, BTW
HTH