You should report this incident to your school's Principal, and/or School District administrators, and see what they do about the incident. If the result is satisfactory to you and yor son (presuming no lasting injury to your son), then you will have to decide whether to drop the matter. If your son shows continuing trauma, then you should seek a local education attorney to discuss your options.
Talk to the Principal first then determine if you are satisfied. The legal system works but should be a last resort not the first action.