Thanks for the answers to my questions and the additional information.
Unfortunately, my state board prohibits phone conversations but I hope that this format continues to be is acceptable to you. My apologies for the inconvenience.
I always worry when a dog this age behaves as you've described and hasn't eaten for four days since these older dogs can deteriorate pretty quickly.
As to possible causes for his behavior, unfortunately, there are number of possibilities:
1. He could have an infection or abscess in this mouth or a bad tooth. Even if you were able to open his mouth, it may not be immediately obvious if there's a problem of not. Sometimes we need to sedate these patients so that a thorough physical examination of the area can be done. Even dental x-rays may be suggested if a tooth root issue is suspected.
2. He could have a systemic disease such as problems with his kidneys or liver. Blood work should help narrow down the list of possibilities.
3. He might be running a fever. If you have a thermometer and can double check it, normal is between 100 and 102.5.
Of course, we'd have to wonder why his temperature is elevated with viruses, bacterial infections, autoimmune disease and even cancer, unfortunately, would have to be on the list.
It sounds as if he needs to see your vet but one way to get more nutrition into him would be what I asked you about: since he's drinking, mix canned food in a blender with pedialyte (which will provide electrolytes) until it's the consistency of soup. If he won't drink it on his own, you can dribble it into the side of his mouth where there's a gap with a small turkey baster or syringe.
I doubt that you'd be able to get any medication into him but Aspirin may help with any inflammation that might be present although I always recommend that it be given with food....obviously this may be difficult since he's not eating. The dose would be 10 mgs per pound of body weight twice a day.....as long as he's not vomiting nor currently taking any other nsaid or steroid medication.
I hope this helps to provide possible explanations for his behavior but it sounds as if a vet visit would be wise tomorrow if not one at an ER hospital today. Deb