[nolefan]

Well, any breeder that would sell you a 6 week old puppy isn't one I would be taking too much advice from. Call your vet back and ask for advice on how much she thinks the puppy should weigh and how quickly she would like to see him gain, what is a good rule of thumb. Try adding a few tablespoons of plain yogurt or plain kefir to see if that might make the kibble more interesting, maybe even a little warm water.

How much does your puppy weigh? How underweight is she? 8 pounds at this age would not be off the mark.
What are you feeding?