My suggestions would be: be loving and accepting, accepting meaning "accepting", never make him feel bad for "not being like everyone else", don't get too visibly upset when he fails to conform to your initial dream of the perfect child, make up a new dream with the real him in it.
Support him later when things get tough at school and he struggles to make friends. Teach him what teachers can't be bothered to try teaching him. Raise his confidence as high as you can. Teach him jokes, magic tricks, tell him about the rules of conversation (your turn, my turn, not too fast, listen carefully, add a related bit of info, move on to something related, etc) and listen to him when he rambles on about a favourite subject, otherwise he'll never understand why he has to listen when nobody listens to him.
Keep an eye out for school bullies, including teachers.
Don't let your family tell you that you should be stricter with the kid. They will, mark my words be consistent, have very clear rules , never mistake a meltdown for a tantrum and vice versa. Let the child rest after he's been stimulated a lot, don't push him beyond his limit.
Things like that. You'll figure this all out by yourself if you haven't already! read a lot and observe your child, you'll know what to do, his diagnosis hasn't changed who he is, and you've known him for 3 years already