Personally, I'd get him eating rat pups, which are close in size to adult mice. One a week is fine, two at a time isn't going to hurt him. I've seen with my two that the rats provide better nutrition, leading to much faster growth, leading to the eating of bigger rats, etc...

Do you feed live or dead prey? Dead is probably safer for the snake once they start in on small rats, though I know some will contest this.