I went through the same thing with my son. We had to move for two years and then returned to the town we had been living in and he went back to his original school. But he had just started wearing eye glasses. the kids teased him about this amongst other things. One kid "attacked" him in the bathroom and otherwise bullied him. I had a discussion with the principal and he didn't really believe my son at first so I just told the principal that I would give him a chance to rectify the problem and then I would tell my son that it is ok to defend himself physically.
The problem continued until one day my son had had enough, got the bigger kid in a headlock, took him down, and delivered some ground and pound. They were both suspended for a day but my son never had any problems with these kids, or any other bullies again. He himself also never bullied anyone nor has he had any violence related problems.
In other words: adults rarely solve these kind of issues. Nearly always it is the kids working this stuff out for themselves. Support your kid's efforts to do so.