How old is your dog?
What is the breed?
Does your dog eat any non food items such as rawhides, bones, toys, greenies, bones, etc.?
Does the dog eat human food, and if so what?
HI , I see you went off line without your information and I wanted to get back to you before I go off line for the night. Easiest things first, If the dog is drinking quickly then that can have him vomit it back up.
If the dog is a pup and not up to date on shots then the parvo virus would be a worry.
If he has eaten any non food items and does not have an appetite and cannot hold down water then I would worry about an obstruction of the bowel and I will direct you to a site for this.
IF the dog eats any fatty human food or may have gotten into the trash then you may be looking at pancreatitis. Symptoms would be fever, vomiting, change in stool, and abdominal pain.
Garbage gut is another possibility if the dog got into trash and will show similar symptoms as pancreatitis.
Obstruction (foreign body)
Garbage gut http://www.petplace.com/dogs/garbage-toxicosis-garbage-gut/page1.aspx