My son had a similar situation, though perhaps a bit more bullying was happening- it's hard to tell. It was awful. At least your son is talking to you; ours lacked the confidence to tell us, as I think it was too humiliating. The bullying consisted of excluding him, laughing at him, moving away from him at lunch- things like that.
I did go talk to his form tutor, with my son's knowledge, though only with his very grudging consent. The way I see it- my son was being bullied, and I needed to tell my son, through my actions, that being mean is unacceptable and that if people are being mean then we do something about it.
My son, like yours, was a nice kid (though annoying as hell sometimes) the kind of person who would put his foot in it to stand up for someone else, etc., also a bit of a know-it-all, in spite of his insecurity. I can understand why other boys would think it was fun to be horrible to him
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His tutor said he needed to talk to our son, and get specific details of things that happened, and he also said that there is always a ringleader (sometimes 2), and that what he would do is talk privately to the ringleader, get him to see how hurtful the behaviour is, just have a discussion. Then if anything else happens, he comes down on the bully like a ton of bricks...
It actually kind of worked. Things got better. Nevertheless, in the end, we ended up deregistering from school for 1 term and then starting a new school. Our son immediately made friends and was never bullied again.
I think changing schools and taking time off was the best thing we could have done- it gave him space to get perspective on school, understand that there is a whole world out there. I didn't know that changing schools puts kids back by 10 months, but it certainly wasn't true for our son, (and the term he missed was the first term of year 10, so not ideal, really, you would have thought).
I am not saying that changing schools is ok for everyone, obviously, just that you might consider it. I think people are quite sexist in the assumptions they make about bullying. Boys don't always just thump one another, or scream violent abuse, and then forget it and play football. They can exclude and tease and turn against their friends too. 