Jeff, you didn't mention his weight, but I think based on his size compared to the mouse in the pic, large rat pups might be your next size progression. Where I live, I have the luxury of being able to obtain two different sizes of rat pups. The small ones I get are about 22-25 grams each, and the large rat pups I get from a seperate supplier weigh in at 30-38 grams.

If you can find rat pups in the 30ish gram range, I would urge you to shift him over.