What should i do about my 2 1/2 month old puppy?

Did wherever you got him not give you some of what he was eating prior? Or at least tell you what they were feeding him so that you can buy the same? Call the breeder and ASK. Don't add the stress of a diet change and the stomach disturbances that come along with that to the stress of a new home. Any diet change needs to be made slowly and after he's settled into your home. If he won't eat the familiar food, he needs to see a vet, as he is likely ill. Small breed puppies also are prone to hypoglycemia when they don't eat regularly, so it's important to get something in him ASAP.