[Felina]

I'd definitely report the situation to the local animal control office and/or SPCA. I'd also contact my vet office or vet offices in the area and see if one of their vets can quickly look over the little guy and talk to the manager about his care.

I'm sure a store manager would listen to a licensed vet telling him the puppy needs medical attention and you'd have a medical vet's opinion in case of any court case regarding this puppy.