If your dog won't eat, take special notice if he is vomiting, having diarrhea, acting lethargic or having any other signs of illness. If he is, call your veterinarian to have him evaluated.
Methods to encourage a dog to eat may include trying different brands of canned food, or small pieces of cooked meat. If he continues not to eat, make sure you call your veterinarian.
Not eating is one of the first symptoms of disease or illness in dogs. For more information on why dogs may not want to eat, please read Anorexia in Dogs