Hi and welcome! I have no advice. I did have a friend whose PG son is AS. He would develop stims (hand waving etc) and they would go away for a time, then return in some other fashion. Just from what you wrote, it doesn't sound like your son's flapping is a stim. It's a concious things he is doing b/c he's amused by it. My friend's son's stims were uncontrollable and I think he mostly was unaware of even doing it. He certainly couldn't control it like your son seems to be able to. Again, I know nothing about this but that is what popped into my head when I read your post.

I also don't think you can go by him being expressive and animated to rule out AS. From what I've read it seems to be more so inappropriate response to things (such as the laughing), inability to read social cues, etc. rather than how the person expresses himself. But I could be totally wrong.