I continually tell ds there is nothing wrong with, in the same way that there is nothing wrong with our youngest having blue eyes when no-one else in the family does. It is just a difference. It makes some things difficult but makes something better too.
I tell him that autism isn't one thing and lots of different people have different kinds of autism. Having autism doesn't make you good or naughty, and both good and naughty people can have autism. It helps that he knows other kids with autism as he went to a special school for a period of time, and now attends a social group for kids with ASD.
I give examples and say 'x's autism means that he finds it difficult to wear any clothes with patterns on', 'you might think that is a bit silly, but you really like talking about tall buildings, which he probably finds a bit silly'.
We talk about it quite a lot actually. We told him in very matter-of-fact ways and in passing and it has always just been generally known until one day he suddenly asked, at a most inconvenient time 'Mummy, what's autism?'
After a very short discussion he wanted to know what percentage of him was autistic. I had to tell him he was 100% autistic and it doesn't really work like that. That for some reason, was the hardest question, and answer for me.