They found a lump in my cat's abdomen and he has been getting very skinny and not licking himself clean much anymore.
The vet knew right away what it was. Apparently it's quite a common phenomenon in older cats and many exhibit the same symptoms and slowly die in this manner.
The vet says they could try and remove the cancer and give him cancer drugs but it would cost thousands of dollars and he is already a very old cat so even if they did all those things, he wouldn't live much longer.
So long as he is not in pain and is still eating then we will not put him down. But we will also not let him suffer a painful death. We bought him special food that is easier to digest and we keep a careful watch over him.