sounds like he has parvo.. hopefully he will pull thru. did you sign him back to the pet store? if you did, that means you cant sue them for the vet bills, and they can do with him as they please. they were very smart in having you sign papers, this way you cant sue them over a sick puppy.. and, NEVER take a petstore dog to the petstores vet, you take the dog to your own vet..
see if there is a puppy lemon law in your state, this way if he is still yours you can sue the pet store for the cost of the treatment as its going to be VERY expensive..just go on msn or google and search for "puppy lemon law" and add your state in to find it..
Lemon Laws for Dogs