That's awful! I hope he's ok. With my DD, she is bullied not so much physically, but verbally. I taught her very early on that she should first say, "Stop it, I don't like it." And if the kids don't stop, she's to move away. If they come after her, she's to run and find a teacher/adult to help her. Anything you can do that will help self-confidence, resilience and social skills will help. Maybe a team sport?