This is absolutely abuse! My own father used to do this kind of thing to us kids. He thought it was funny to thump us on top of our heads or hold us up by our necks with our feet dangling. Then when we were crying or choking and wiggling to get free, he'd laugh at us and make fun. It was awful the things he did and it was abusive. Of course, we didn't understand that at the time and let me tell you, there has been life-long fallout. Five kids, and all of us suffer from depression or alcoholism. I so wish someone, neighbor or family, had stepped in to help us.
OP,I think you've made a good first step. Hopefully, the principal will do a thorough investigation. And if this were my son, and I believed that what he was saying is true, I would consider calling the police. They will maketheir own investigation. If there is nothing going on, the case will be dropped. If there *is* inappropriate behavior going on, the court system will act on it. This teacher needs to be held accountable. What happens if the school lets him go? He moves on to another school and continues his warped style of teaching.
There is never a good reason to hit a teenager on the back of the head, or anywhere else. There is never a good reason to curse or humiliate anyone, regardless of age.