Pictures will definitely help. Often underweight BP's will have a triangular look to them, with a prominant spine. He may just be young too, or a slow grower. If he pooped, he's eaten before.

Anyways, it's a good idea to get his next poo and take it to a vet for a fecal float to make sure that there are no internal parasites living in the little guy.

And yes, some snakes like to eat butt first. It's not a problem usually. Next time, just try to make the head of the mouse warmer.