I agree with
@Langenschwert that there are no easy solutions to bullying, and am not a fan of zero-tolerance policies in schools. The topic of bullying comes up periodically, and I have gotten the impression over the years here that my opinion is a minority one. I really regret how the situation is continually framed so that bullies are bad kids and the bullied are good kids being victimized. I think this is a very damaging and typically ill-considered one for all of the kids involved, so I apologize in advance if I sound like I'm on a soap-box. I'll borrow some of the language from my previous posts on the subject, so if this sounds familiar, forgive me.
First, my opinion is that all kids are incomplete human beings, who are all learning to be happy, healthy and productive adults, any discussion that involves kids needs to consider the welfare of ALL of the kids, which includes the bullies. It's perfectly understandable that we only tend to advocate for the kids we're connected to (our own or those whom we coach.) I think it's important that we all remember that schools have a responsibility to advocate for all of the children, not just ours. And it's possible (or probable) that the kid we think is a little angel, isn't. And conversely, the kid we believe is the devil also isn't.
All kids are on some kind of a spectrum of dysfunction. They are unfinished adults. If they exercised good judgment and sound decision making skills, we'd be kicking them out of the house at 12 and not 18 (or 20 or maybe even 30).
Also, kids bully each other in many ways, and if they don't have the proper tools to deal with conflict in a healthy way, they will learn to survive in an unhealthy way, which is what we generally define as bullying. But, it's not just physical. There are kids who bully others intellectually and emotionally. While a physical altercation may SEEM pretty clear cut, the kid who appears to be the bully may be the victim, who is standing up to a bully. Point is, sometimes, the "scrawny" kid who is punched in the nose is the bully, not the victim. The big, strong kid might be the victim. This is true now more than ever.
While we all have an understandable tendency to believe our kids' versions of stories without a lot of analysis, my experience has been that most fights go both ways. When you put a bunch of kids together, there are countless interactions that are completely unobserved and it's difficult when your own kid is involved, to remain objective. The unfortunate consequence, however, is that you might be reinforcing negative behaviors in your kid.
So, once again, I'll recommend to the OP that you focus on building skills. Consider your role to be the same as a baseball coach or a guitar instructor. Give the child a safe place to learn something and be a part of a group where he can be himself and let his guard down a little. Give him some unqualified support, along with realistic, appropriate feedback so that he starts to develop some self confidence and resilience.
Anything more than this, I would recommend you avoid unless it's in your lane and you're asked to do so by the parents.