How to treat a malnourished puppy?!

Supplement him with goat's milk (unpasteurized if you can find it) if you have to. Cow milk is totally inappropriate.
Supervise feeding at least 4 times a day. Put him on a back nipple (nearest the hind legs) where most of the milk is and keep the other bigger puppies from knocking him off.
This is a very common problem when there are more puppies than nipples, but this is how professional breeders do it.
Allowing dogs to breed indiscriminately is a bad idea. Breeding takes a lot of time, effort, money and knowledge, and most pet owners aren't equipped to deal with it. Lots of dogs and puppies suffer and die as a result of unprepared, ill-educated owners.

Vet "treatment" is completely unnecessary and expensive. If there is some kind of pathology (which is doubtful) it's unfortunate, but you'll end up paying through the nose for nothing, because no amount of veterinary treatment will save it. An honest vet would be up front about this and tell you they can't influence the outcome.

Go to the nearest animal shelter or SPCA facility, they will have had experience with this problem, make it clear that you are only looking for advice and want to keep and do the best for the puppy. Advice like this is usually free but you may want to make a donation to the organization. Should this fail you may think of taking the puppy to a vet.

Let him have mother's milk sometimes instead of the others and give the others store milk instead sometimes. If that doesn't help, have the vet check him out.

Take that puppy to a vet today.You really should get them all
checked out including mom.

Treating a malnourished dog takes patience, lots of care and a special diet. It is also important to have a veterinarian check out the dog thoroughly for any other health problems, such as parasites, that could be affecting the dog's weight.

Keep the dog in a comfortably warm area. With little body fat, malnourished dogs are unable to keep themselves warm, and the act of shivering actually burns calories that a malnourished dog cannot afford to burn. If they are severely malnourished, considering putting a portable heater near the bed.

Feed the dog breakfast first thing in the morning. Make a mixture of equal parts of scrambled egg, white rice and high protein dog food. Feed at least 1 cup of this for a small dog, 2 cups for a medium dog and 3 cups for a larger dog. If the dog will eat a little more, give them another cup. It is important not to overfeed all at once, as this could cause the dog to vomit.

Fill up the dog's dish with high protein dog food at lunchtime, and mix in a raw egg. Follow the same feeding guidelines as you did for breakfast to determine how much to feed the dog.

Cook up your dog a nice dinner consisting of high protein dog food, white rice and eggs. Eggs can be cooked or mixed in raw.

Put out a bowl of dry dog food for your dog for overnight for him to munch on. Do not add any food to it that should not be sitting out, such as eggs.

If the dog won't eat, try rubbing honey on his gums. Honey stimulates hunger.

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