You cant just feed rat pups. They dont have fully developed bones and they dont have too much calcium. If i were you i would throw in an adult mouse. Also you can try a weaned rat they have a better bone density and they have more fur which is high in calcium. I have read that as a Ball Python grows they need calcium to develope strong bones and scales. I suppose its good that he is eating at all but maybe feed a rat pup and a mouse. The rat pup would be high in protein because they have more meat on them this is good for you snakes muscles and the adult mouse has fully developed bones and a full coat of fur this gives them strong bones. Or again maybe go up in prey size. And since you have so many rat pups its a great excuse to buy another baby ball python.