Turn off the T.V.. Discourage sedentary entertainments (except for reading, of course.) :wink: Make sure he has access to physical play, instead. Depending on your area and your neighborhood, you can provide him with all kinds of alternate entertainment: A basketball and hoop; a bicycle; a trampoline; roller blades; a climbing rope tied to a tree branch.
During the dark days of winter, and bad weather, indoor activities at home can be done with dvd/video. My grandson has a kids yoga video and a "tai chi for young people" that he uses daily. The catch? An adult has to do these things with him to keep him engaged. He doesn't maintain interest or focus on his own. That's true, of course, for many activities with younger children. Do them with them, and they are enthusiastic. On their own, and they lose interest. Let him see that you value physical activity over sedentary activity yourself, and he'll value those things as well.
Don't forget organized sports opportunities. Soccer, basketball, swimming (if you have an indoor pool locally.)
Finally, remember the part that diet plays. No fast food, and avoid over-processed foods. Fresh foods, with plenty of fresh growing things and whole grains, avoiding foods with any added sugars or sweeteners, and foods made out of processed flours. Drink plenty of fresh water, and leave sodas, juice "drinks," and juices themselves behind. Fresh whole fruit to eat, instead of fruit juice to drink.