Giving a length for size is rather meaningless. Get a digital kitchen scale and find out how much he weighs in grams. I'm also not sure what you mean by "small sized frozen mice" - are you feeding pinkies, fuzzies, hoppers, small adults, or ? Because if you're following the advice of the big box store where you bought him (I found your other threads) then he's very likely been underfed.
This time of year BP's tend to start feeding if they didn't eat much over the winter. Yours may be just hungry. If so this would be an ideal time to make the switch from mice to rats - offer one f/t mouse as usual, and then while he's still in "food mode" follow up with a warmed f/t rat pup.