Hi Greensleeves, I do know how you are feeling. I was really worried about telling my 7 year old ds about his dx. (He was given a verbal dx of Aspergers in January - although we have since been told that this dx is quite likely to change. )
Its a toughie, I really didn't want to say anything that would make him feel weird or different, but then again he already does feel different, so what we were trying to do was reassure that it was ok.
It sounds like we approached it in a similar way to you. We sat him down and talked a bit about the spectrum and how different people are at different points along the scale, with him being very near the top end. We also said that basically Aspergers is just the name of the man that first understood children that had struggles and problems like ds has and that some of the worlds cleverest and most brilliant scientists etc had Aspergers.
We explained that everyone in the world has some difficulties and problems in some areas of their lives and being dx with Aspergers just means that we and the doctors have been better able to identify which things he struggles with, which is a good thing as it means we and they can help him. We tried to focus on the fact that, now we know what the problem is, we can help him to improve on or even overcome some of those problems and hopefully he will be happier and more relaxed as a result.
That was about as far as we took it at the time, but he has asked a few more questions since.
He has asked if it is an illness and we said it isn't an illness, its just a different way of thinking and experiencing the world.
On the face of things he seemed to take it quite well, but he tends to internalise worries quite a bit, so I think we will have to be the ones to keep up the dialogue and make it just a normal part of every day life.
I suppose it will take a while for in to sink in and process and then he may come back with more questions. I really don't know what the best way to handle it is, I guess we just have to play it by ear and keep reinforcing the positive slant as much as we can.
There is a factsheet on the NAS websitehere about telling children about their dx, but to be honest I did't find it all that helpful personally.
Sorry, not much help really, but wanted you to know you aren't alone.