Go straight to the principal or vice and let them know what you saw. It is unacceptable that he is doing this. Yes, he needs pulling up now while he is still young. He might have a hard background but it is no excuse for hurting others.

He can be asked to sit in the foyer when he gets to school til the bell rings. Has the principal or vice been made aware of his behaviour to others or has the teacher just handled it?
Sometimes the teacher does not let higher up know as they think they are not doing their job correctly