That group on 3 or 4 boys who regularly get into trouble, all schools have that kind of a group. Changing school won't likely escape the existence of this kind of group - they'll just be different people.
The fact that one of the boy has had a previous sanction in relation to treatment of your son is a good thing. It means he's on the bullying log. So if he reports again, the school can escalate. He needs to report as things happen tho. Specific incidents - what happened, when, where, who was involved. Report it swiftly so it can be investigated.
Incidentally, schools can (and often do) investigate without disclosing your son reported it. Stops the fear of being labelled a 'snitch'. They may say the incident was seen by a third party, or seen on CCTV.
Resilience work may not be relevant. But school deal with many parental reports of bullying that on investigation do not meet the definition of bullying. In such cases its important your son doesn't feel invalidated just because the incident isn't bullying. It doesn't have to be called bullying to mean it was an absolutely awful experience with others.
For example you talk of one boy being horrible and getting a sanction. Then another time a different boy did something. That incident with the second boy may not be bullying, especially if its a first reported incident. It doesn't invalidate how he feels about it tho. And if it does feel to him that because its not defined as bullying then it means noone cares, that indicates he needs some resilience work.