If it’s a safety concern or he’s getting physically hurt, report electronically but also call the school bullying coordinator (there is a list online- I think it’s usually the head counselor, assistant principal, and anyone else you can reach and be very explicit that it’s a safety issue.
If it’s verbal bullying, report to the school- repeatedly if necessary- but also work with your kid on how he responds. My kid struggled socially and a fair bit of his social skills class was focused on responding to bullying. Ignoring by sitting quietly is not always the best way. It’s often better to turn to a friend and start a conversation about anything else. Come up with an excuse to leave. Call them out on it- why are you always on me about x, it’s gotten really boring. Or have them think of a few insults to snap back. Know your kid - my kid freezes and could never smoothly snap back, but it works well for some. Learning to deal with jerks is an important skill that everyone needs.