Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
Your guy may be suffering from an Upper Respiratory Infection which is causing this. I would urge you to find a rectal thermometer and take his temperature. if this is over 104, then this is an inefction that will require antibiotics by your vet. If this were a digestive disorder, there would be vomiting and diarrhea.
There could also be a metabolic disorder causing your guys discomfort so this will require treatment by your vet.
For now, all you can do is nursing care which will consist of Pedialyte, 2-3 ounces every 4 hrs to keep him hydrated and keep his electrolytes in balance and give Nutrical every 4 hrs. This is a high calorie supplement that can be purchased at the pet store and will give him the nutrients that he requires to get past this.If this is an infection, there is a chance that he can fight this off as long as his body is competent to do this. If he does not respond to this, he will require veterinary attention.
Sincerely, Dr. Loretta