Thank you, everyone.
sis - gosh, it's a bit rough that it was taken out of your hands like that! I'd have been quite annoyed with the paediatrician, I think.
jimjams - I think we're already past the point of telling him so early that he won't remember being told - in fact, I think we were probably past that point by the time he got his diagnosis, at 3.5, as he does have an amazing memory.
monica2 - it is exactly a reaction like your dd's that I fear. He already compares himself unfavourably to his younger brother, who is a brilliant communicator and makes friends very easily. I'm afraid that he might kind of lose hope and stop trying.
scummy - that is really very very helpful - it's very interesting to get a bit of a wider perspective on this. I don't know anyone in RL who has an older HFA/Asperger's child, so I can't get this sort of insight directly.
Anoah - yes, quite, how would we explain it exactly? I'm not sure whether my DS1 perceives himself as different or not. I've tentatively asked him about it and he's indignantly refuted me, but his isolation seems to be growing more profound, not less so.
Think I need to stock up on some more books. Am going to get the Stanton one on living with HFA, I think, and have a look at the NAS catalogue.
Thanks again folks.
I think on balance, having read all your replies,