Sorry to re-open such a horrific thread but I did want to say, based on the pictures that a tape worm could be to blame for the size. I rescued a BP about a month ago that was suffering from belly burns, tape worm, and pin worms. The vet estimated his age to be 18 months to 2 years, however because of the tape worm he was about the same size around as a 6 month old snake, and he was eating 2 adult mice every 4-5 days. Sorry I don't remember what he was treated with, but it was a 1 time injection, and then a oral med for the pin worms. Since being treated for the tape worm, he has put on 100g and has moved up to rat pups, and is entering his second shed this month.

I know wild caught snakes from petstores can have a variety of different parasites, and that might explain the small size.

I have no idea what would cause the mouse to get stuck like that, but I find the whole situation disgusting and heartbreaking. That animal should have been to the vet already, If it were my snake I would have been packing him off to the vet within an hour, not posting 2 days later because the problem still hadn't fixed itself!
Truly heartbreaking!!