
Take the pup to the vet, I have had this with one of mine, only it wasn't putting on any weight, at 2 wks old it only weighed, 9oz, I was at the vet on the Monday, Wed and Fri and he gave pup an injection each visit to see if it would help. Pup is now 3 wks old and weighs 12 oz, which is still very small, but at last I can see a fat belly and he is getting "rounder" instead of the skinny little thing he was, the other pups in the litter now weigh over 2lbs each.
Its really quite funny to watch him with the rest of the litter he is the most agile runs away, and just pushes himself inbetween the rest while they are lying sleeping.
I have also had the problem of mums milk drying up and not wanting to feed them most of the time, so I have had to wean them early, but since I have done this the little one has come on leaps and bounds he is lapping out of the dish with the rest, I made up a puppy porridge with JWB puppy, I made it soft with boiling water them put into the liquidiser to make like a porridge and add some lactol and they are loving it. I found the puppy pieces did not soften as much as the adult.
He still will not be going away at 8 wks though I will keep him here as long as necessary until I know he is 100% o.k. with vet approval. mind you because I have eye dropper fed him wrapped up in a t towel like a baby in a shawl, I dont know if I can let him go to anyone else, its just like a bond you have with a new baby especially as you are up every couple of hours during the night as well feeding. I am really shattered though, but in the long run its worth it.
The vet also mentioned to me fading puppy and I know he still is not out of danger at 3wks old , every day now is a bonus.
Hopefully your pup will be o.k., but do take it to the vet.