Is there any chance he has been exposed to any rat or mouse poison?
Did he eat any bones?
Did he eat any table food?
Can you look at his gums and see if they are pink or pale?
When you look at the gums, touch them with your finger and watch them to see if they turn white and then back pink again, and let me know how many seconds it takes for them to turn pink again.
No poison, bones, or table food. His gums are pink. When I touch them they turn white and turn pink within a second.
Okay. All that sounds good -
Has he had three parvovirus vaccinations at ages 6, 9, and 12 weeks old?
With a young dog like yours, the most likely things that cause these symptoms are: parvovirus, pancreatitis, ingestion of a bone or sharp object (toy), or a toxin.
If your dog was given vaccinations by a veterinarian for parvovirus on schedule, most likely this isn't the case. If you are sure he hasn't ingested anything unusual to cause GI tract irritation, then he probably was exposed to something toxic. In any event, and especially with his small size, I would get him to an emergency vet tonight. He will dehydrate quickly with vomiting and diarrhea.
It is a good sign that he has nice, pink gums and that indicates he hasn't lost a great deal of blood yet.
Please let me know if you have more questions.