Do you live in Florida? Is he store-bought? It's unethical to purchase a pet and then let them fend for themselves. You need to take responsibility for the animal that you purchased. Natural sunlight is one of the best things out there for chams, but it also depends on temperature and climate. Even though veiled chameleons are surviving in Florida, it doesn't mean that they are happy during the cold snaps. They are solely surviving and passing on their genes. Any animal hobbyist's goal should be for their animal to thrive and not just survive.
He is getting skinny because he is not in his natural habitat. There is not enough food. You need to capture him and bring him back inside, or keep him in an outdoor cage, so you can provide him adequate food.
When you own an animal, it is your responsibility to provide them with the best care possible. This is not simply letting your cham fend for himself and potentially starve. If you are not up to the task, you need to rehome him.
In regards to "fattening" him up. There is no reason to feed fatty foods. You need to feed him a good staple feeder such as crickets, dubia roaches, and black soldier fly larvae and cycle in hornworms, silkworms, and some super worms. If you give him a good insectivorous diet, he will regain weight on his own. Feeder lizards are high in protein and too many can cause gout.
I will give you the benefit of the doubt and assume you are not a troll. If you no longer care for your animal, you need to rehome him. If you do care for him, you need to capture him and put him back in an appropriately sized cage so you can give him the proper amount of food.
