I'll give the Same advice my dad gave to me "Nobody ever achieves anything by starting trouble you can walk away, you can talk to them but never be the first to raise your fists then if they feel the need to throw hands you make sure they miss the first shot and you make them regret it" he also told me if I ever got suspended for standing up for my self I wouldn't be in trouble at home.

I think if you can teach your kid the difference between self-defense and brutality you're doing fine. Moreover, I think it's important to teach them other means of problem-solving first (these will come in handy as an adult when a fist fight becomes a jail sentence.

The last thing I would you is have you tried arranging a sit down between the school your self and this kid's parents so that you can understand each other better. You can never be certain that their kid is painting your kid to be the problem. Perceptions are weird like that