Welcome.. and good for you for rescuing a poor ball python.

First off, read our care sheet. It covers all basic aspects of ball python husbandry: http://www.ball-pythons.net/modules....warticle&id=52

Going to a reptile vet is a great choice. If you need help finding a reptile vet (dog/cat vets are not going to be much help here) go to this site.. it's a directory of reptile vets. http://www.arav.org/Directory.htm

How is the tank set up now for him/her?

EDIT: oh I forgot to add that a ball python can live for a VERY long time without eating. Most likely it hasn't eaten because it has been stressed by being in an improper environment. Is it possible for you to post a picture? Then we can tell you how underweight it is.