I'm having some trouble feeding the appropriate sized meal to my BP. He is about 10 months old now and is still on adult mice that I get from the pet stores. I feel he is underweight and undersized for his age.
He is about 24inches long and weighs 310 grams. He hasn't grown a lot in length since I've had him.

The mice that i have been feeding him are roughly 20-22grams which is only 6% of his body weight. I had picked up a box of 50 rat pups (25-30 grams each) from big cheese rodents, but he has absolutely refused to eat them. I've tried them on about 10 different occasions without luck. The rest are just sitting in my freezer.
What could I possibly try to feed him that is more appropriate for his size and so he grows better? The petstores don't carry any mice that are bigger than the adult mice.

I even tried the small rat they had at Petco a couple times but he doesn't even look at it.