This puppy needs to be seen by a vet ASAP! We can't really diagnose over the internet what is wrong with this puppy. Has the mother pushed this puppy away, does it feel cold? If it's been pushed away you need to bottle feed, or make the mother nurse this puppy. Make sure this puppy nurses without competition, take the other puppies out of the whelping box while this puppy nurses. Also make sure this puppy stays warm. I believe there is something called fading puppy syndrome, or something, where the puppy will begin to feel cold. Don't know exactly because I never bred though.