My DS has started doing this as well in the last month. I copped a nice one on my shoulder the other night, owww. He seemes to do it like your son, when he is excited/hyper and it is usually in the witching hour before bedtime. I just stop what we are doing, sit him on the floor and tell him no, that hurts. I hope it is just a phase. oh and he only does it to me as well, so far not DH.