My brother has HFA and has been playing basketball for around 10 years (he's 21 now). He loves it and gets a real buzz out of it. It was quite difficult for him at the beginning and anxiety is still a problem for him today (he puts too much pressure on himself to score highly in every game). When he gets anxious, he tries to calm himself down by deep-breathing.
Basketball has definitely helped him socialise with others and learn how to work as part of a team. I don't have any specific advice, but I just wanted to say that, despite the issues involved (particularly when he started), basketball has really helped him in a lot of ways. I'm sure that, as your son enjoys it, it'll have positive effects for him too. My brother's even gone to Shanghai and various places in the UK to play with the Special Olympics, which was really exciting for him (and us!).