The most common cause of this gastric upset is a parasite problem, especially in a dog this age. You need to go ahead and see the vet with a fresh stool sample so that you can get a fecal float and an appropriate dewormer. Giardia and coccidia are also concerns with new puppies.
Until you can see the vet, I would go ahead and switch him over to a bland diet of only boiled chicken and white rice. This will help to settle his stomach.
Let me know if you still have questions.
