One of my peeps at one of the houses I work at will do this, albeit very lightly. He's partially verbal; he can say "ok" "yeah" and "good night," but is not able to express how he is feeling verbally. It's not an issue for him as he pretty much cups his hand and gently taps his chin. For your son, I'd suggest redirecting him to something soft or calming when he is doing this, so that he learns to use that as an outlet. Hope this helps :)