My son is 10 years old, now. When he was that age, he banged his head on the floor for kicks. It freaked me out. He would kneel on the floor and bang his head, briefly cry when he hit it hard enough, then he would do it again. I asked my dr. at the time if there was SOMETHING wrong with him. And....his diagnosis.....just like you are saying.....self expression! Plus a little dare-devil - he'd do it until it hurt! At first, I would immediately stop him, but it gave him the startling reaction he was looking for. I tried distracting him, even saying, "OUCH" the floor is crying!" At some point, it really stopped. I have talked to other moms of boys (my daughter NEVER did this stuff!) and they had the same story. I CAN tell you your son is normal - time will cease this habit. Just stay on it and try different things - it is frustrating to watch, isn't it? Good luck!