9 month old banging his head

Ri

My 9 month old has had issues with banging his head it made it worse when he had to where a cranial helmet. I think he does it to stimulate and we try stopping it. The biggest issue is the highchair he bangs his head when he eats. Has anyone else experienced this and what do you do? The pediatrician isn't concerned but we are for the share force he does it. Highchair has a pad but it isn't enough. Anypointers?