My 4-5 week old pit bull puppy won't eat, what to do?

my husband and i brought home a 4.5 week old pit bull puppy from a neighbor and he said the puppy was on hard food and that the mother already stopped feeding him, when we brought him home he had an enlarged belly which is now gone but he stopped eating, we tried to ask his previous owners if we could let him see his mom and they weren't home and they won't be for a while. we're not sure what to do, please help.