It's pretty darn impressive what they can do. They haven't survived thousands of years for nothing though. And they've survived all these years with no arms or legs.
If it were me, I'd stick to the large mice for a little while. Then I'd try to switch. Maybe feed him the mice until his 3rd shed with you. Then wait him out. Dont feed durong shed or after the first aign of shed. Then let him get good and hungry (by this point he'll be plenty healthy enough to miss a week or two) and then offer a rat.