I third the martial arts. What I noticed is that kids usually are there for a reason, so your son will be in good company. They may not be there for the same reason, but, at least where we are, there is pretty good social training along with it.
Also, about social stories, there is plenty of material for these online and even books written about it. Early on, I found pictorial social stories for young kids online that looked very useful, though we were beyond that stage. I bet there is even more now.