I would continue feeding him the small rats until he looks like putting more weight on. But stay away from mice, they don't have the noutrishinal(sp) value as the rats do. If he want take the rats try scenting a rat with a mouse. I would also try feeding him a "large" small rat and see if he will take it. With any luck you can get him to eat medium rats. Mine is on medium rats and growing like a weed. Hope that this helps.