It's really a difficult situation and I understand your frustration. I'd first tell you to document every incident (dates, bruises, witnesses) and request another meeting with the principal by bringing your evidence - sometimes you really have to be pushy to get things moving. At the same time, see if your son can get closer to other kids or adults during recess, like a teacher or supervisor, that can sometimes be enough to discourage bullies. If after that the school still doesn't do anything concrete (meeting with the parents of the other kids, consequences, accommodations), then yeah you should consider changing schools, because it's not okay for him to get beaten up without anyone actually doing something about it.