I think, it would be a good idea to offer your son to go with us to the gym.Don’t tell him that he have to lose weight.You have to be careful with this because it is a problem theme for teen.You can hurt his feelings this way.Try to cook heathy food, it normalizes his nutrition and his organism will lose weight slowly.Go for shopping with him,it will make him happy.Try to talk about his appearance,maybe he will tell you what is his problem then decisions will be easier