"I think bullying was too strong a word tbh, between these boys the other kids coat was hidden, his lunch box thrown across the hall, he was chased around the playground."
"One of the bullies bit him"
These actions are bullying. If your son came home crying to you daily saying that a gang of boys were hiding his clothes, throwing his lunchbox around, chased about, bitten etc, you'd be fine with it would you?
You haven't even taken the time to consider how shit that kid feels, every day having to come into school wondering what he'll face today. Being singled out and picked on by a group is horrible and every boy involved should be thoroughly ashamed. Bullies like your son will only get worse if they're enabled by parents whose darlings can do no wrong and minimise their shitty behaviour as "boys will be boys" rather than taking responsibility for the fact that their child is making another child's school-life incredibly difficult.
OP as someone who was horribly bullied in school by a gang of kids at a similar age to the boy being bullied by your son, it has always stuck with me. I'm sure, like you, the parents of my bullies thought I was making it up too as no teachers witnessed the behaviour and it was simply my word against theirs.
I would have felt so much safer and happier (and in turn my education would have improved) while attending school if my school had taken the same action as your son's school clearly have.
Whether you want to admit it or not, you've raised a bully or a child that is so desperate to fit in with bullies that he will bully to make himself seem "cool" and rather than diminishing the actions of your son and his little gang, you should take this opportunity to teach him a valuable lesson - actions have consequences. If he doesn't want to be spoken about in front of the class, he should learn to be kinder, not to bully those who he has singled out as a target. You should get him to understand how the other child feels and how he would feel if his whole gang did the same things to him.
Justifying your son's mean actions but saying that the other child said "you'll never catch me!" What else is he supposed to say?! He's a young child being picked on daily and you're putting the blame on him for staying strong in the face of bullies? It really is scraping the barrel for excuses.
While your son and his gang aren't beating this child up on the daily, it still doesn't mean that these actions of being singled out, picked on and laughed at won't stick with this child for life. They'll be damaging his self esteem. At this age children are just figuring out their place in the world and your son and his gang will be telling this child that his place is as an outsider. That sort of shit stays with you for life and I can tell you that from first hand experience.
It is not an admission of poor parenting to admit that your child is in the wrong, but it is poor parenting to write it off as "nothing" and make every excuse under the sun to let your little darling continue with his behaviour towards others. Teach him not to bully, don't turn on the teachers who are doing your job for you and allowing that child to have a school environment which is safe and free from bullying. Once your son and his gang have shown that they have grown up enough to behave properly without singling people out to pick on, I'm sure they'll be allowed to spend time together again.