It is a really difficult situation. In an ideal world yes you’d be able to tell the school and they would be able to immediately resolve it.
In reality, 13/14yo old’s who are going to bully are not going to be deterred in the slightest by a teacher, and it gives them the added ammunition of then “mummy’s boy”, “grass” and the like. There’s also the consideration that at 14 years old depending on the school, there could well be times when no teachers are present and it leaves him vulnerable at those times- lunch breaks, bus before/after school, online/social media.
He shouldn’t have to put up with it and it’s incredibly difficult, you can work on this at home, potentially look elsewhere to see if there is another school, or it may be worth speaking to the school to see if it’s possible to just keep them apart as in diff classes/breaks if that is possible.
I would speak to your son about it, support him either way and let him have final decision on whether to involve the school or not. There is a risk to reporting it that it just makes things much worse and it is ultimately your son who will have to live with those consequences so it needs to be his decision really.
I really feel for him (and you), kids can be evil and it’s so difficult to know what to do.