He's undersize for sure. Don't panic--in the wild they're equipped to deal with a wide range of feast and famine, so he'll come up to size if you increase his intake.

But you do want to increase his intake. Try offering more than one mouse per feeding, or switching to larger mice. If switching him over to rats, try leading off with small mouse, then following up with a rat pinkie or fuzzy.

When offering more than one, ideally you want to wait until just after he's swallowed the tail of the first mouse, and drop the next food item into view as soon as he flicks his tongue. ("Pounding" I think is defined as putting the next one in his mouth before he's quite finished with the tail, and letting the swallowing reflex take over?--I know this technique works really well, but I don't like doing it with my animals. I figure if they're healthy and they really don't want to eat more, then they probably have their reasons.)