School-wise, you should have a private meeting with the school's guidance counselor to let him know what happened so he may privately and discreetly let all your son's teachers and coaches (this should NEVER happen again in gym or any sports activity once the athletic staff is notified!) know how they should handle this emotional issue.
I am sure doctors may have suggested a healthy exercise and eating (NOT DIETING!!!) program can reduce your son's overall body fat and lessen the breast-like appearance of his chest. He could also gain self-esteem and fitness by learning and living a new lifestyle as it relates to food and his body.
He needs to feel good about the things he does do well and enjoys so make sure you offer him encouraging words often about any accomplishments or progress he is making in ANY area of his life. This may seem impossible to ask of him, but if he could learn some quick, comic comebacks that included the fact this is a condition he can't eliminate by just loosing weight, it could help him through the uneasy times he is teased. Also, sometimes responding to teasing by simply saying something like," You know, if making me feel terrible about myself makes you feel better, take your best shot- Maybe you'll feel terrific!" can stop a bully or teaser in their tracks because then they know they can' get a big rise out of you.
Good luck. I know your help and support along with some consciousness-raising on the part of others (and perhaps that exercise and eating program) will go a long way towards minimizing the hurt he feels.