Please show the dog to your local vet. He may have mild fever. Make sure your dog is being properly vaccinated and dewormed. Try not to take the puppy in public parks or any place or object which is frequented or touched upon by other dogs. This will only make sure your dog doesn't pick up infection.
Please always keep clean drinking water all the time. Make sure you don't feed milk to the puppy as dogs are lactose intolerant which cause stomach upset.
You can feed him curd or buttermilk which is good for stomach.