You should talk to the principal. Ask them for specific

measures to ensure your son is not hit again, e.g., make sure your son is not staying at the same room with the other boy. I do not think you have a case to sue them; but, you can report to state department that regulating daycare. (Before you decide to report, you'd better find another daycare for your son first.) Honestly, if I were you, I would have found another place long ago. (There are cases that school let go the kid who causes too much trouble.)