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. There are some fun cookbooks in the bookstore specifically for young kids. You can control the food coming into your house so that there are not temptations that would undermine your efforts. You provide a wide variety of healthy, low fat foods.