Look to his diet. Many additives are feminizing and processed food is a big offender. Also his being overweight is more suggestive of poor food choices than it is of overeating. Children who eat wholesome natural foods seem to eat all day and never get fat.
I suggest working with your son's MD on an appropriate diet. Never place your child on a diet before consulting with an MD.
Also, it is extremely important to weigh your son's feelings concerning his weight and whether he truly wants to lose weight. If your son is depressed as you say, this could negatively affect any weight loss program. Your son probably should be evaluated by a mental health professional to address the depression. In turn, this might increase his desire and motivation to lose weight should this be his goal. The school counselor might be a good resource for this support.
Ya, this will not be easy as this has been a lifestyle for him and you are asking him to change it. I assume he is not on any medications that can add to the physical appearance problems?
I agree with Mark. This is not something that you want to attempt by yourself. At this age, it is not an easy thing to undertake. I am sure his pediatrician is concerned. This is something that I would think has to be gradually achieved. No crash courses. Get the help of a dietician. And you really may want to consider an evaluation by a mental health professional as his depression may be a factor in the weight gains.