You don't want to fatten him up with fatty feeders. (Imagine trying to gain weight with fast food!) Use healthy feeders with the fatty ones as treats.
Good choices include crickets, roaches, superworms, silkworms, hornworms, butterworms, soldier grubs, mini mealworms (t. obscurus) etc. Feeders that should be more like treats include mealworms (t. molitor) and wax worms. Superworms also shouldn't be used primarily, but they can be used fairly regularly. It's just best not to use them primarily because some chameleons will go on feeding strikes where they will only eat superworms. Try not to let any one feeder make up more than 25% of the diet if it's possible.
EDIT: Also make sure that everything that can be gutloaded, is! Make sure to use a variety of healthy veggies/fruits like mustard greens, collard greens, dandelion greens, endive, escarole, kale, squash, carrots, yams, sweet potatoes, sweet peppers, strawberries, apples, oranges, berries, papaya, etc. Plus use a high quality dry gutload.
