It's easier to switch trouble makers if they're feeding off tongs. Sounds like you just leave the prey in the enclosure and he eats up the mice? If so, start offering off tongs with a bit of action until you get him striking/constricting regularly. Then it should be easier to pull a bait and switch. Make sure the rat is the same size as the mouse. Scent the rat up real good with the mouse. Offer mouse as usual but stay just out of strike range - using it to get him into feed mode. Offer scented rat in strike range. Alternate between the two, keeping him interested with the mouse, until he hits the rat.