I hope this pup isn't eating solids yet. They aren't old enough at this age to digest solid food, and it can be very harmful to them. If this is the case, I would get pup to the vet, and there they would probably do an enema, and give you some laxatives made especially for your pet, (do not use human laxatives unless the vet says to) and check to make sure that nothing else is obstructing the pups bowels. Hope this helps, good luck.