I’m so sorry OP and so angry on you and your son’s behalf. Applaud the fact that he is able to not retaliate physically - am not sure if their bullying is physical or more verbal stuff?
Does you son have good friends at school and/or in the hobby activity? Am assuming he does as he is invited to birthday parties etc. Does he have friends he spends time with outside school one on one? Are you friendly with any of their parents?
I agree with PP about staying on school and hobby leader to try and get them to actively stamp out the behaviour. It’s terrifying that they don’t work harder to protect kids in their care.
someone else might have a way better/more informed perspective on this than me but my instinct is not only to pursue those in authority in these setting to intervene and protect but ALSO to model for him ways to take back control and create a certain level of self protection… I appreciate it’s kind of a fantasy view in some ways but the truth is that all through life we meet people who bully and we can’t always change them or avoid them. But we can always take protective steps to limit their ability to damage us. It’s a really shitty lesson for a 9yo to have to learn but… So what I am getting at is things like…
don’t suffer in silence - tell your allies what’s going on, encourage him to do that same. Show him that HE has nothing to be ashamed of and that this isn’t something that needs to be secret/doesn’t reflect on him in any way.
ask for their help in supporting and protecting him - even simple things like parents of his friends of his finding something to praise or make a positive comment about when he is playing at their house. Spending time with family members who will do that same. Does he have a couple of friends who would spend playground time with him as a plan, so that he can more easily avoid the bullies. Ask their parents to also talk about it and explain why it’s important. Make it a collective responsibility to help keep your child safer.
role play with him - what do they say, what can he do? Say back? Practice, have a laugh, be silly - but also be dead serious about walking away. Try and find a stinger or two.
Make it clear to school that these are your tactics and you don’t expect to get called in if the other boys complain he said ‘X’ to them (obviously X shouldn’t be verbal abuse - you are aiming for the art of the searing put down).
Also ask him to think about why are these kids doing this? What does it mean? He currently thinks (as is totally natural) that’s it’s because HE is or does x,y or z. But try and get him to talk and think about why HE wouldn’t do these things to another kid and why these kids are… (clue - they are miserable, insecure, have horrid home lives, don’t feel that people will like them if they aren’t tough, toxic masculinity etc etc) What kind of adults will they become? What kind of adult does he want to become?
You don’t need to throw punches to defend yourself, but you do need to build, have (or fake having) a cast iron sense of your own ‘rightness’ and the absolute unimportance of anyone else’s view about you.
This may require you to take actions which make you really uncomfortable. If that’s the case then use friends as sounding boards, and role play yourself to practice. You don’t need to be aggressive, confrontational or pick a fight - you’re trying to model being assertive and no-nonsense. But would the hobby leader or school facilitate a meeting? If something happens/has happened and you see the parent at drop off or pick up then raise it directly but calmly and be specific about the incident.
Bully’s think their victims’ fear will protect them from reprisal, because it won’t get talked about - disabuse them of that notion. Parents will say ‘boys will be boys’, but in the end if they do the ‘seeming nice’ act then they don’t want to have public showdowns about their kids behaviour as it makes them look bad, so they might actually tell their children to lay off if only for a quiet life (and probably the children actually don’t want to be in trouble with their folks - where do you think they have learnt that this is ok?! Probably from their even more aggressive parents!) Make sure you have another adult with you if you approach them, so you have a witness (and they have a reason to maintain their image).
If there are parents who would not invite those children to a birthday party because they are nasty bullies and they’d rather have your son there then ask them to do so.
I’m not saying start a vendetta, or make empty threats. I’m just saying - use every tool at your disposal, and every reasonable person who could provide self esteem boosts for your son and back up for both of you.
Don’t just tell your son to stand up for himself, let him see that you’re standing up for him too and that you, in the nicest possible way, accept no bullshit. Make it clear that, when someone is doing this to one friend, it’s not enough for other friends to turn a blind eye - they should be actively protecting (not by starting a fight but by sticking close). And let him know that, although he shouldn’t have to, there are things he can also do to take control like not straying too far from the adults in the playground who are (supposed) sources of protection.
None of this might help straight away, but this will stay with him and you want him to know there are tools at his disposal to help him, if not stop it, at least survive it.
Sorry bit of an essay and not trying to be prescriptive - apologies if it seems that way. But seeing you galvanize support for him, even if it’s not obvious to the bullies, will stay with him… (also bet he isn’t the only child they’ve done this to, so you might find some parents very willing to empathise). Good luck