I have the same issue with my 3year old. It has improved with paying close attention to his needs, if his needs are met, he usually won't do it. I have had to hold his head to keep him from banging it on anything, I tell him he can't hurt himself, and he calms down. It is about attention sometimes. I feel if you can give it give it, but if you are busy, putting him in a room alone does help.