I think bumping up the prey size or feeding two mice that size might solve your problem. I really see nothing else wrong with the setup.
The snake is eating regularly, correct?
Yes he has never refused food. However the next size up that my pet store has is just huge, but I will take a picture of those too as my sister's corn snake eats them. We are trying to breed the two rats we have right now though, so should have any size would need soon.