Hello Liz, my name isXXXXX and I'll do my best to help you today. This is, indeed, a very difficult situation...for you and Baghera both, actually. There could be several different explanations for the behavior you describe, even though you might think it woudl only be secondary to one disease:
Inflammatory Bowel Disease
or cancer, I am sad to say.
As you probably know, blood work and urinalysis would be needed to diagnose most of these conditions.
Of course, some of these conditions might be considered “better” than others. For example, we can treat diabetes with insulin and/or diet. We have very effective treatments for Hyperthyroid Disease and many of these cats can do very well. It can take up to 75% of kidney tissue to be damaged before changes are seen in the blood so this is not as good a disease to have; treatment is often focused on preventing further damage.
Inflammatory Bowel Disease is often controlled with drugs and diet....sometimes easier said that done, depending on the cat.
And, of course, cancer is a bad diagnosis, no matter how you look at it.
When it comes to situations like this, I often advise owners to consider the quality of their cat's life.....
Do they still enjoy what they used to do?
Are they still eating and maintaining their weight?
What is their general attitude? Are they still active or are they lethargic and depressed?
Obviously, if you know that Baghera wouldn't tolerate any sort of treatment or if you wouldn't stress him out by attempting to treat him, then knowing what sort of underlying disease is causing his symptoms becomes moot in a way.
But you might consider a change in diet if you haven't already on the off chance that Inflammatory Bowel Disease is responsible for his gastrointestinal signs.
Feeding grain free may help in some cases but often you have to feed more hypoallergenic diets such as Z/D (from your vet), or Natural BaXXXXX XXXXXmited Ingredient Diets, Nature's Variety Instincts line, Evo duck or venison, Nature's Variety Frozen Raw Medallions (I recommend that they be zapped in the microwave for 10-15 seconds on each side).
Also, probiotics such as Forti flora can often help a lot of these cats; these are available on the internet and some pet stores.
Pepcid AC at a dose of 1/4th of a 10 mg tablet twice a day can often help with the nausea although I realize that is isn't always the easiest thing to do...pilling a cat twice a day.
Obviously I can't tell you what to do but if you at least schedule an office visit, it's possible that a good physical exam will detect a problem (such as small kidneys or an abdominal mass) which then might make the decision easier.
I will say that if blood work is normal in a cat with the symptoms you describe and this is absolutely new behavior (he's not had a sensitive stomach in the past), then most likely the diagnosis is going to be cancer in a cat this age, unfortunately. Euthanasia may be the kindest option under these circumstances.
I hope this helps but I'm really sorry that you and Baghera are having to go through this. Deb