My son is getting bullied

My son is 10 years old and this entire school year he has been getting bullied. The kid will push him, trip him, smack things out of his hands and just make his school life a living hell. Last week the kid pushed my son onto the ground and kicked him hard enough he left bruises. I went to the school and talked to the principal and the teachers and they did absolutely nothing. This isn’t the first time i talked to them either, I have even talked to the kids parents and nothing has happened. Last week the kid pushed my son again and I have no idea what to do to make it stop. My husband tells my son to just fight him back next time it happens but I don’t want my son getting in trouble and suspended. Please help. I have no idea what to do next.