Incidents of eating disorders are on the rise for teenaged boys. You are right to be concerned... One thing to look out for is the amount of TIME he spends exercising. Is it cutting into other things, like homework, spending time with friends? Look into sports training, etc., and remind him gently about allowing his muscles to recover because recovery is as important as the work out.

You may also want to watch out for what he's reading, like muscle building magazines and books. Watch his calcium intake and his protein intake and if necessary, humor him by switching to lower fat milk, more lean protein (skinless chicken breasts, lean beef) and providing more fruit. Make sure he's getting all the calories he needs to sustain the exercise and his growth.

But you can also help by perhaps having him visit a nutritionist and a sports trainer - professionals who may be better able to give guidance and to whom he'll listen because they are (1) experts and (2) not his mom!