I have thoughts on this but not the time right now. Probably later tonight. Has kitty been tested for heartworm? Very possible if your location is still correct. It’s often mistaken for asthma and can be ruled out with an in-house blood test that takes about 10 minutes. The drooling after medicine is because it tastes bad to him. Prednisolone is most prevalent in tablet form, I’m surprised your vet gave you liquid. You can get small empty gel caps and put the tablets inside to pill him that way, so he doesn’t taste anything bad. Ask your vet if you can get tablets for the pred instead. Your vet might even have empty gel caps, and if not you can get them at any health/vitamin shop. Size 2 or 3.
Watch him for yellow coloring. It’s easiest to see around the thinner fur in front of his ears, also the whites of the eyes and roof of mouth. The biggest risk of a cat not eating is liver failure. It’s encouraging that his bloodwork was OK earlier this week, but he’s at risk with continued anorexia.
Sorry, I can’t offer any advice. We had two cats; Jasmine was a big, black cat with a huge tail and very friendly, he used to wake me in the morning by scratching my chest, Gemma was a small tabby sweetheart who was timid, she’d try to hide as much as possible but occasionally just lie on her back in the hallway, asking for a tum rub. When they were in their ‘prime’, maybe 6 or 7, my dad became allergic to cat hair - I did too, a few years later to very fluffy cats - we gave them up for adoption to a nice home. I still remember walking out the front door of that house, seeing them at the top of the stairs looking at me in bewilderment and confusion. I was in tears.
So I’ve never lived with an old cat. Do they know they’re old? Jasmine quickly got that bringing me a bird he’d nearly killed was not a good idea, he was smart and I had to put the bird out of its misery. A friend had a dog that developed epilepsy after being hit by a car, and his personality changed like he knew he wasn’t a young pup anymore. He was a good dog, very protective but would back down from any confrontations in the park. He still loved to chase rabbits but steered well clear of the roads. He had epileptic fits in his sleep but was okay, otherwise.
Ziggy’s had 15 good years, is giving him another 1 or 2 painful years a good thing? He probably knows he’s really sick. A house visit to end his pain seems the most gentle.