To help hyrdrate, I would also recommend a soak. I got my BP from Petsmart and he was also in bad shape when I got him. He was underfed, underweight, and had a prolapse. But he's thriving now. :)
To soak, I use a large plastic tub with holes I drilled into it, I fill the bottom with about a half inch of lukewarm water and put Sev in there with the lid on. The tub gets nice and humid and he enjoys swimming around. I use a clear tub and just cover it with a towel so he feels safe. I did this once a week for about a month until he was better.