What do I say to this ANNOYING little boy next to me on a bus?

That is unacceptable. You need to get up and tell that child's parent to tell their child to stop or else you'll report him to your family. Small annoying children are a nuisance and parents who do nothing don't deserve to be parents. Just don't hit the child back. Tell your family so they become a witness if i'ts that serious. If this continues I suggest you tell your family to do something.