Hi,I have a suggestion.Could you start meeting him from school on the occasional afternoon?This way the bullies won't know if or when you will be meeting him and the cycle may get broken.My son was being bullied as well and one way we dealt with the going home session was to send his elder sister to occasionally meet him from school.One of her looks was more than enough a fcouple of times a week to frighten them off and eventually the pattern at homecoming was broken.Possibly because they moved onto someone else.
The problem of course soon stopped the moment they realised my son was going to be over 6ft tall and built like a brickhouse in year 10.No comfort to your brother in the meantime but it will pass.In the meantime I think your mother must keep up her efforts if she feels it will help.Trouble is by the time she gets somewhere it may well be too late as he will be leaving school.
I also found that the HOY was a tremendous help and was prepared to meet and speak with me any time because he knew that my son was the victim not the bully.Sometimes it seems to me some HOY's fail to grasp this very important fact(and I am speaking as a teacher here).If you can get the HOY on your side it does help.