My 6 month old puppy won't stop vomitting?

He started throwing up last night around 11 pm, and has been steadily ever since. It isn't foamy, and there is no blood. He doesn't have diarrhea, and he is alert and attentive, just seems tired. I haven't fed him since the first time he threw up, and any amount of water causes him to vomit. He has urinated throughout the day, but I'm still worried about dehydration. Is this just a bug, or something more serious?