What is the best diet for an overweight athletic teenager in rehab?

Exercise & lower cal. There is no one right "diet" for every overweight teenager. Clearly exercising 4-6 times a week aerobically is key various recent studies have compared low fat and low carb diets and found when adhered to both cause equal weight loss . Given that fact, many teenagers have diets very high in excess carbs- french fries, donuts, candy, pasta and cake. If you do, then a carb restricted diet is best.
Low glycemic index. A five meal per day mediteranean diet program which includes low glycemic index fruits and vegetables works best in my opinion. An athlete can use a high quality whey protein mix for mid morning and mid afternoon snacks. I prefer a shake versus a bar but either way eating regulary will provide the necessary fuel for energy and will not be hungry. Consider a bia test to find out his caloric needs.