Okay thank you for the additional information.
It's probably very likely that being overweight is affecting him emotionally, though it's good to hear that he does enjoy certain activities still. He is lacking motivation though and you are right that he needs to exercise and eat well.
He really needs to full a sense of success that will keep him motivated. Sometimes kids do well with a team sport that gets them moving, or in finding an activity that is fun for them. Often the gym does get boring for many people and goes unused.
Obviously the bullying at school did not help his self esteem and he may have turned to food for emotional comfort, and so the problem got worse. You could set him up with a reward system, where he is given praise and rewards along the way for reaching weight loss goals. Whatever you and he decide on together that he would find motivating...be it monetary or material or just a special night out, etc. Rewards always work better than negative reinforcement. Also, the school change might be a really good boost for him emotionally to start anew and become more motivated.
What I would suggest is that you have him see his physician (just his regular doctor) who can get him on an exercise and eating plan tailored to his needs. I would also recommend that maybe some counseling could really benefit him just to increase self-esteem and reaching goals, etc. If you can give me your zip code I can find some therapists in your area who specialize in weight loss, and your insurance name if you have insurance.