Are you feeding him a fuzzy mouse or a fuzzy rat? A fuzzy mouse would be way too small---if you want to feed mice a hopper mouse would be the right size for him. Between 8-12 grams is the proper size (10-15% of body weight). Fuzzy rats are about that size too. How much did he weigh when you got him? Is he gaining steadily (doesn't have to be much, just steady)?