Make sure he is getting plenty of fluids, definitely don't starve him as a pup can soon get dehydrated. Feed him some cooked chicken and rice, but also get him checked by the vet. What are you feeding him, and have you changed his food recently?
also mix a bit of egg in his food because it helps it could be that bowl syndrome thing that we get my old dog had that 1 moment she could go an the next it was like a fountain p.s dont stave ur puppies for a day