Hi Carmen, I am sorry your dog is sick.
If you can afford to take him to a veterinarian RIGHT NOW as an emergency, I would do that. He may have a condition called hemorrhagic gastroenteritis, which can be treated with fluids and medications. However, he may have something else wrong, like if he ate some mouse or rat poison, he might have a fatal bleeding problem. Even with just an exam, the vet can also help you decide if it would be in his best interest to euthanize him - while this is never an easy decision, sometimes it is the right one to make if the dog is otherwise suffering and the treatment is long and expensive.
if you cannot afford a vet, I would call your local SPCA or Humane Society and see if they can at least look at him. I would take him there right away if they say yes. They may be able to assess him for you and help you decide if he can be treated, or if the best decision would be to euthanize him. Some shelters also offer this service to pet owners.
Good luck.
Dr. Tanis!
Dog Veterinarian
10 years experience in small animal emergency medicine