hmmm... check his nose, if it is dry, it's possible that your dog is suffering from parvo - a viral illness that affects dogs.
kindly see a vet immediately. depending on the severity your dog may need IV (dextrose)
here are the most common signs of parvo:
-fever
-diarrhea (usually with blood if the infection is severe)
-vomitting
-lack of energy
-loss of appetite
-rapid weight loss