I'm sorry the boy was hitting you in school. It sounds like a very traumatic experience. It was clearly inappropriate and unacceptable. Is there someone you can talk to? Your teacher, principal, school councilor or parents (whoever you are most comfortable with) would be a good person to start with.
As far as brain damage is concerned, it's rather unlikely. If you're concerned about this you can always make an appointment with your doctor and get his/her opinion. In the meantime please go talk to someone in authority at school. You shouldn't have to live through this anxiety and depression. You don't deserve it.