Gorta,
Ds has this autistic friend, quite severely autistic. Let's call him A. DH is aspergers.
We started by saying from time to time that people are different. Diff hair colours, skins, tastes in food.
And then , next stage, that some people can do some things brilliantly and some not at all. But everyone has is good at something, though it may take time to discover it or not. Eg, I am fat but good at literacy, DH is thin but good at maths. You can see we are fat and thin but the other attributes take time to discover..
When Ds came back saying why does A do x, we said that it was because his brain worked more like DH's and he was very good at x, but he found y more difficult. We pointed our the Ds was really good at maths but less good at eg running and again people are different.
We then had to take Ds to hospital frequently, but a visit to the Dr is just like that, and said that people's bodies were made differently, just like their minds. And some people had feet that came out a diff way from normal ( two children in our street) but they were no different really from anyone else, they just needed a bit more help with their feet just as Ds did with his literacy and running and A did making friends or stopping himself stimming.
And we pointed out that A was nice and interesting and knew a lot about Henry Vlll and Chelsea, whereas friend B knew masses about Man united and Ds himself was an obsessive gunners man. But A would find it hard to understand others, just like B cannot sing in tune. And you need to support A if he us surrounded by people and looks unhappy, or give him space, or give yourself space, if you need some, but not forget that A has qualities and us just as good as you and B. And if B is being picked on by the music teacher, be nice to him afterwards and say Ferguson is the best manager on the world.
Does that help?
Later on we explored more, through the London Eye mystery, ace book by Siobhan Dowd, and later on still the curious incident.