The best way is to exercise and eat well with him. Make meals a family event-Cook, Eat, and Clean up together. Take walks/bike rides together-It's getting nicer out. Help him track progress with you. It doesn't have to be pounds and inches lost but feeling better and being able to run/jog/walk/bike a longer distance/time.
Get him used to looking at nutrition labels on food and explain how it works, protein/carbs/fat/ etc. You will see in no time he will get into the numbers game with you. My 11 yr old has been into it since last year and is pretty up on it. Show him what good eating is, but don't make a big deal out of it. He will soon catch on, and like Lithium said get him active, try unplugging the nintendo this summer.
-------------- We must become the change we want to see.