Deacon, it's never a wasted visit to the vet when it comes down to the health of your pet. Your veterinarian has diagnostics and medication to help your baby feel better.
Small pets, under 20#, are subject to something called hypoglycemia. This is known as low blood sugar. If your baby isn't eating, his blood sugar can drop rapidly. Above all, if he becomes tired and barely responsive--you need to take him to your vet promptly. Vomiting can also lead to dehydration, it doesn't take very long with small, young pets.
What could be ailing your pet is a long list. Young ones are subject to chewing and swallowing things that they shouldn't which can upset their stomach, get stuck in their intestines, etc. Intestinal parasites can also cause an upset stomach, sometimes vomiting accompanies. The parvo virus is another worry for owners of young pets, especially those that have not received 3-4 DHLPP (combo) vaccinations when they were young puppies (usually administered at 8 weeks and every 3 weeks until about 5 months of age).
If your pet does not eat or he vomits today I would highly recommend taking him in and having your vet do an examination to see what could be ailing your baby. Sometimes it's an easy fix with a few injections and some observation, sometimes its more indepth with a few days of hospitalization. Only the vet that examines your vet will be able to determine exactly what it is that is making your pup sick.
I hope treatment is quick and easy on you both. Good luck!
Sami