Why do you want to fatten him up? If he's given a good diet, he should eat what he needs and be a good healthy weight. Some younger dragons can look a little on the skinny side but fill out at they get older. Just mentioning it because intentionally adding weight to a healthy animal isn't a healthy thing to do. On the other hand, if he has been sick or something else has been wrong which caused him to be underweight, then yes, offering more treats like wax worms (use sparingly though, they're quite fatty and can be addictive), horn worms, silkworms (both fine to use as staples as well) etc. can help encourage him to eat a bit more and gain weight a bit more quickly.