I really think that you need to talk to the school on this because it sounds like these boys are escalating their behaviour towards him and it need to be stopped now before it gets to the point where he is refusing to go to school and is visibly affected by it. You need not tell him that you've done it because he will probably only worry about it more. I think the school would appreciate you telling them about it now instead of later too. It might be easier to stop these boys now, instead of later. I hope it does stop soon for him though, poor little fella