Thanks so much for the additional information.
Gum color sounds normal which is a good thing as is the fact that he's not reacting when you press on his abdomen. When a male cat becomes lethargic, I always worry that they may be blocked and can't urinate. This would obviously be considered a medical emergency since it can be fatal. However, most cats are quite uncomfortable/painful when you press on their bladders so this is less likely to be his problem.
1. I do understand about taking his temperature although if you can manage to get your hands on a thermometer to take it (if he'll let you, that is) may be very helpful. Normal is between 100 and 102.5.
Fevers can cause cats to suddenly behave as you describe.
Of course, then we'd have to wonder why the temperature is elevated. Explanations for this could be: a bacterial infection, a virus (such as leukemia, AIDS, FIP, or an early upper respiratory infection), or toxoplasmosis.
Since he has such a strong odor emanating from his mouth and he's so resistant to your looking in there, it's possible that he has an abscess or infected tooth which is causing his issues.
There is also a condition in cats that is called Fever of Unknown Origin--which is exactly what it sounds like in that that we can't find an explanation for the elevated temperature. Treatment is primarily supportive which consists of fluids and drugs to reduce the fever. Most cats recover very quickly once drugs are initiated.
2. Pancreatitis can happen very suddenly; these cats just stop eating and can become lethargic. We don't have a good explanation for why this happens, either.
We do have a test for this (spec fPL) although I've come to doubt its reliability in some cases; treatment consists of fluids and pain medication (we believe the pain is why they stop eating).
3. Other systemic diseases such as kidney disease, liver disease, auto immune diseases, problems with the heart, etc. can cause these symptoms although usually there are other signs such as increased thirst or urination, weight loss, vomiting, etc.
I would love to be able to advise you to give him over something over the counter but
it’s not considered advisable or safe to do so since cats are so sensitive to many of them and they could cause more harm.
However, I share your concern about dehydration. Maintaining hydration with water or Pedialyte is important, especially if there’s a fever. You can dribble this into the side of the mouth with an eye dropper or syringe every 30-60 minutes even if he won't easily open his mouth. If you approach the side (not the front), there's a gap in the teeth into which you can provide the liquid.
Given the extent of his lethargy, it might not be a bad idea to have him seen tonight if you could. I'd rather be safe than sorry in a case like this, especially if there's any chance that he might be blocked.
But, if he seems to rally a bit, especially after you get fluids into him, then it may be acceptable to wait until tomorrow to have him seen.
I hope this helps and that he's feeling better soon. Deb