You may be lucky, though in being able to instill in him a change in food preferences and exercise habits that he will take home with him. One of the most important things you can do this summer is to help him become a more active boy. No weight loss can occur if it is not accompanied by an increase in activity. Before he even steps into he house you may want to establish some rules about the TV, for example, no TV except in the evening or an hour or two on a particularly rainy day when outdoor activities are restricted. Exercise should be fun, and not be for the sake of losing weight.
Summer is a great time to increase a child's love of vegetables.
A great project would be for you to all plant a small vegetable garden together. My son, who hated cooked peas, would walk through our garden and harvest and eat raw peas right on the spot. Nothing beats the great taste and texture of fresh raw vegetables. The same holds true for meal preparation. Let him help you in the kitchen to prepare the food.