A.Z.
I am going to assume that he does this mainly at night...if so(I guess you can even try it during the day) put a pair of socks on his hands-some may say that is mean...My one daughter ALWAYS had her fingers in her mouth(wasn't a thumb sucker or anything)and after a couple of times of her wearing socks on her hands-including when we went outside or to the store-she stopped..good luck!