I wouldn't worry too much about it if he is eating well for you, he will catch up eventually. Keep in mind, a lot of people feed their snakes a lot more than they would be eating in the wild to bulk them up faster. It's not all that uncommon for a breeder to go really light on feeding of things that aren't holdbacks, and as long as they aren't getting too thin, it's not going to hurt anything. If anything, it's probably healthier to some extent.
He probably is capable of fertilizing a female, but I absolutely wouldn't try this season. He just doesn't have enough to loose if he goes off food from breeding, which they often do. He might anyways smelling females in season, but if he isn't actually breeding, he isn't going to burn nearly the amount of calories as if he were putting in all that work. Would likely be a death sentence to him to put him with a female. If you are wanting to use him to motivate another male, throw one of his sheds in with the ones you are trying to breed, the smell of him might be enough to get the other one kick started. I imagine the rate the small boy is putting on weight he is shedding pretty frequently for you.