It sounds like your family is going to be in for some big changes. I know that may not come easy, and you're going to have to be firm and persistent. It's going to be a big job for you to change how you shop and cook, but I think you'll see rewards for everyone -- all of you will feel better. You'll see better behavior and health with the kids in time, which will help all of you throughout your lives. This will be one of the best gifts you give your kids, to adopt a healthier lifestyle.
I strongly suggest that you explore the Dr. Sears LEAN site - it has really good nutritional info:
This may help you become aware of ways that your diet may have been contributing to your son's problems, and may help you see some places to start making changes.
I think you need to begin by purging the junk out of the house, and stop buying anything that's full of sugar and artificial ingredients (generally, any processed foods). Go as natural as you can - if it grew that way, it's ok to buy. If it's manufactured with ingredients you can't pronounce, leave it at the store. Packaged foods are almost always full of sugars, fats, salt, refined flours and preservatives. Better to find fresh, natural foods that offer nutrition instead.
Don't buy soda or koolaid or any other sugar drink, period. Buy milk and that's it. If you really want other drinks in the house, maybe make fresh iced tea or get juice that is 100% juice (preferably not apple because that's essentially nothing but sugar). Stick with milk or water.
My rule is (pretty much) one-a-day. One treat: if it's a soda, that's it, or if it's ice cream or cookies or what have you - but those have to be considered 'extras'. My kids generally can have a soda about once a week or less. If you are buying boxed cereal, you may have to switch to healthier kinds. Or switch to natural oatmeal and cook it from scratch, and add your own cinnamon, brown sugar, etc.
It sounds like you are going to sort of have to start from scratch. For lunches I would think you could make healthy sandwiches and offer some fruit, salads, maybe trail mix... I know these are out of your son's repertoire now, but he is going to have to change some habits. Just don't buy chips, doritos, gatorade, and other packaged dessert items. For snacks, sometimes I'll make oatmeal cookies with chocolate chips. Yes there's chocolate in them, which the kids like of course, but the oatmeal is healthy, and I can substitute whole wheat flour and reduce the sugar in the recipe without them knowing.
I don't know what you typically make for dinners, but I would think in terms of a meat or protein component like grilled chicken, fish, etc, plus a vegetable (could be cooked veggies or salad or even cut up raw veggies arranged on a plate... you may be surprised - when offered in different arrangements, kids will try them because they look good); and then a carb. You'll need to be careful with this. Potatoes, rice, pasta are the usual standby's in our country, but there are so many other, healthier grains available.
Will your family eat meat loaf? chicken? fish? BBQ? pasta salad or other salads?
A lot of times while I'm cooking, I will cut up raw peppers (red or yellow), cucumbers, or other veggies. When my daughters wander in trying to snack before dinner, I hand them that bowl - they will munch away, and that way they're getting something healthy. Sometimes my little one comes home hungry and I'll give her trail mix - same thing. It's something healthy so that if she's full and won't eat dinner later, at least I know she got something good in her.
Back to the carbs, whole grains have a lower glycemic index than our common processed ones, and will be better for your son if diabetes is an issue. These can be found at a health food store and generally cook up like rice, with different cooking times. We sometimes cook quinoa, millet, cous cous, amaranth, and others.
Another option is polenta, which is basically yellow cornmeal. There's no need to spend $3-4 on premade rolls you see in the store - just buy some coarse yellow cornmeal. To make that, the proportion is roughly 1 cup polenta to 4 cups water, stirred really carefully so you don't get lumps... If you cook up a batch to the consistency of thick oatmeal, and let it cool off in the fridge (overnight is ideal) - then you can slice it up and use it in meals that way. I actually cook up slices of it in a little bit of butter, and when both sides are brown, put a little salt on it and it's great.
I can send some specific recipes if you would like. We do a lot of vegetarian meals here. Good luck, and let me know if you would like some recipes.