My son bangs his head since newborn now he's almost 4 and he still bangs his head. He functions normal otherwise please help?

Temper tantrum. Most of the time children persist in head banging (or other temper tantrums) because it works for them- it gets them what they want. Stop responding to the head banging! walk away and don't give in to the behavior. When he calms down and speaks appropriately, then give him your attention (but don't give him what he wanted). When he finds out it doesn't work, he'll stop banging his head.