Thanks for your response. Maybe I should've explained this a little better - he keeps his fingers straight with his hands pointed towards each other, then moves all of the fingers up and down very rapidly. Like wiggling, I guess, but the fingers are straight instead of bent. Sometimes he rocks his body forward when doing this.

This is only one of many stims. Sometimes he spaces out when he's stimming and misses what I'm saying. Then I have to repeat everything. I've asked him about the stimming but he doesn't really seem interested in talking about it, and I'm not sure to what extent he's aware he's doing it. The most I've gotten from him about what it feels like is that he says it makes him feel "free".