If it's stimming, redirection is best. If you try to eliminate the problem it just breaks out somewhere else. It's like trying to build a dam with mud bricks. You plug a leak here and the pressure makes the wall break down somewhere else. it is better to let the water flow under your control, than to have the entire dam fail.
However, hitting himself sounds like a stimulant that is not one to try to keep. He maybe needs someone to talk to him about them, ay "Some of these are okay to do, as long as you try to avoid disrupting things for other people. But please try to keep the impact down if you can."
Not all stimming is obvious - the classic hand flapping that people associated with autism is often because they watch their fingers as they flap them in front of their face, and the flicker of light through the fingers is what seems to soothe them. But for difficult child 3, he could just look at the flicker of light through the trees and it would soothe him. He never flapped, ever. But he would stare entranced at a tree. Or anything else that flickered.
Marg
