Keep only healthy foods and beverages in the house.
Don't use chemical sweeteners. They're really unhealthy. Try stevia, instead. I like KAL brand the best; you might find another brand that you like better.
As the others have said, keep him active.
Make sure he's drinking plenty of water and other fluids.
Make sure he's eating nutrient-dense foods. That way, he won't have so many cravings. If he's craving sweet things in particular, he might be dealing with a calcium deficiency. Don't give him extra dairy to make up for it. Dairy-based calcium is very poorly absorbed. Go for plant-based dairy, or just supplement. Make sure it's an absorbable form of calcium - The Now brand of calcium citrate powder is great, and you just add it to a drink. Doesn't taste like anything. It's also cheap per dose.
Also, don't limit his eating times. This can slow down his metabolism. Actually, let him eat whenever he wants, but watch his portion sizes (not too closely, but I have a hunch that his stomach is a bit stretched out, so he feels hungrier than he needs to; this will change pretty quickly with generally smaller portion sizes) and also keep an eye on what he's eating. If he doesn't eat breakfast, start insisting that he eat at least something (healthy, of course). Breakfast is an acquired taste. Skinny people tend to eat breakfast; overweight people tend to skip it. It's the most important meal of the day.
Don't let him eat much at night, or it'll just sit in his stomach and get absorbed all through the night instead of running through his system, getting burned off.
Make sure he's getting lots of fiber.
Make sure that he has quick access to healthy things. Chop those carrots into carrot sticks! Make things easy for him to pull out of the fridge. Think: if every day he substitutes just one bad snack for, say, a huge bowl full of watermelon, he'll peel off the pounds. (Let him eat all of the fruits and veggies he wants. Really. Yes, even the sugary fruits. You just can't gain or keep weight eating tons of fruits and veggies.)
Watch the fat in the salad dressings. Secret diet killer. Same with mayo.
Keep his meat, milk and egg intake super low. Animal protein makes the body sluggish. He will get plenty of protein everywhere else. Protein is packed into everything (except fruit). Vegetarians tend to be able to eat pretty much whatever they want and still stay slim. This is why. It's about metabolism, and having all that extra energy. Even just sitting in a chair, you end up burning way more calories.
Don't worry about sugar. Look at his fat intake. That's where the real problem tends to be.