That MOUSE looked just right to me.
He looks to be in great condition to me. I raised a number of my females on mice only (2 mice a week).
I have switched all of mine over to rats, not because I felt that rats were any "better" than mice (there are tons of healthy mousers being kept in pet homes all over the country), but because I chose not to breed mice, and breed rats instead.
If you'd like to do so for convenience for you, he should be fine on a small rat (I'd say 4 to 5 weeks old). Start with one equivalent to or slightly larger than the mice he eats now in size.
He's a beautiful animal!