Luckily he is still in the predisposed state and you have time to teach him new habits. Right now you can only try and get him to follow your advice when he is out of your house. I'd suggest having a really serious talk with the grandparents. Tell them that they aren't showing him love by permitting him to harm himself. Make it a really serious conversation. If there are diabetics in your family, they must know how bad the disease can harm folks.
If he has a sweet tooth try getting him to eat fruit when he wants something sweet. Make him fruit smoothies, kids love them. Make fruit sorbet in a machine you can buy and let him know that you want him to have that treat.
Fill the house with healthy snacks. There are a lot of sugar free and low sugar foods around now.
Get him a form of exercise he will like. I've heard that the Wii games can help kids get moving, and I'm sure he'll love getting something special. Maybe sign him up for an exercise class he likes. Karate is popular with young boys.
Mostly make sure he knows that eating well isn't a punishment! Let him see that you hope he will make better food choices because you want him to be a babe magnet, not a sick kid.
It might work.
