5 month old pit bull puppy won't eat , drink's very little water, vomiting. What could be the problem?

It sound serious and why would you wait to take him to the vet and waste time here asking. Do you not have any sense of urgency seeing this happening to your young dog and the pain he must be going through??

Parvo Virus
Allergies
Upset stomach
Worms
Intestinal blockage...

The possibilities are endless really without a vet running tests.