Do you know if your dog ingested something? Was he eating grass. You can try some pepto about 1 Teaspoon, wait an hour see if he eats, if he vomits again, call the vet, constant vomiting is a sign of pancreaitis.
-Call your doctor if it happens several times. Sometimes they eat something that disagreed with them. If they thrown up a lot throughout the day, it could also be the liver. if your dog ingested something, like a toxin, it will affect the liver and cause your dog to constantly throw up. This is what my vet told me today, because my dog is having the same problem.