He was taken from his mother too early. Puppies should never leave their mother before they are 8 weeks old, not just for their physical well being but so they can be properly, completely weened and socialized. There are just some things in life that a puppy can only learn from other dogs, chiefly their mother. Puppies taken too young are more prone to behavioral issues later on in life.
You need to bring your puppy to be examined by a vet. Most breeders require you to do this within a short time frame of getting your dog but you clearly got your dog from an uneducated backyard breeder, which was a large mistake on your part. He needs to be examined and get his proper shots. If he is not drinking then that could be a real problem. Dehydration is very dangerous for young puppies.
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