Of those foods you serve, allow him to eat how much he wants. If you have served a dinner of Grilled skinless chicken breast, steamed brown rice, boiled baby carrots, whole wheat rolls and a salad with a lite dressing, there is no reason why your son should not be allowed to eat what he wants of those foods. You can feel happy that he is filling up on nutritious, low fat foods. Also allow a healthy dessert, such as a fresh fruit salad. It will be a major help to your son that he likes sports.
Weight management is doomed to failure without regular exercise.
Encourage him to continue his involvement in them. Engage the whole family in an active lifestyle. If you don't already get regular exercise, begin to now as a role model for your son as well as for your own health. Plan family activities that involve exercise. Rather than staying home to watch a video, go out for a bike ride, or rent a tennis court at a local health club, or invite a friend of his to go bowling with you.