There are many reasons why a dog would vomit this much. The most common include gastrointestinal obstruction with a foreign body (such as a sock, or a piece of a toy), intusussception (obstruction of the intestines when they telescope in on themselves), gastric dilatation and volvulus (twisted stomach), toxic ingestion, gastrointestinal infectious disease (viral, bacterial, etc..), kidney disease, liver disease, pancreatitis, or a simple dietary indiscretion. Differentiating these illnesses requires a physical examination by a veterinarian and blood and urine laboratory testing, plus abdominal X-rays.
Since your dog sounds as though he is still well hydrated, and he has only just begun vomiting within the hour, I do not think that you need to rush him to an emergency clinic immediately. I would just keep your eye on him closely for the rest of the evening. Do not offer him anymore food or water tonight. If he makes it through until the morning with no more vomiting, then you can offer him a few tablespoons of water in the morning. If he holds this down, then offer him a few more about 2 hours later. As long as he keeps the water down, you can then offer him a few tablespoons of a bland diet. I recommend boiled white rice and boiled white meat skinless chicken breast, mixed 3 parts rice to 1 part chicken. If he holds this down, you can offer him another few tablespoons of it a few hours later. Slowly increase the amount of this bland diet that you are feeding him over the course of 24-48 hours. Once you feel that he is back to his normal self, you can gradually mix in his regular diet with the bland diet until he is fully back on his usual food.
If he does continue to vomit tonight, then he should go into the emergency clinic to be evaluated for any of the problems that I listed.
I am attaching a client information sheet that I use in my practice that discusses this problem in more detail. I hope that you find it useful.Click Here
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.