Oh gosh, I always get a bit worried when I hear that cats aren't eating much. We lost Paddington in the fall and it was really complicated and started with hyperthyroidism that was unfortunately overmedicated which made him lose his appetite. He just nibbled here and there but because he had been such a big cat and lost so much weight (which lead us to get him to the vet and then the thyroid issue was discovered) fatty liver disease developed. I didn't know enough about it and by the time I did, I was up every 2 hours trying to feed him (syringe) but it was too late and we lost him. So now when I hear a cat isn't eating I always suggest trying everything under the sun to get them going again sooner rather than later.

I know Fister's t-4 was checked and it's unknown what might be causing him to drink and eat less and maybe it's just the new normal as someone else suggested. But if you're starting to feel his bones, I would maybe try and get him jumpstarted again (if possible) by picking up some high calorie wet food like 'Reovery' or Hills A/D from the vet. You may have used the A/D in the past but if you haven't, it's got tons of calories and is made for cats that are quite ill so it's very digestable and super palatable. It's blened so much that it is like a 'whipped' pate and you can add just a bit of water and mix it and then you can syringe it into Fister. But I know you mentioned that he hates it so it could be tough. But if you can get some good, dense calories into him, it might get him back on track and feeling well enough to start eating more on his own and hopefully drinking more too.

I was also thinking that some chicken or beef baby food might interest him. Paddy was keen on it one day but not the next so you never know. Depends on how they're feeling I guess.

Anyway, just trying to come up with ideas to help. Maybe he is just slowing down in all areas of life including eating and drinking but it does seem that if he's losing weight, maybe some underlying thing is going on. Have you had him checked for diabetes? I was just reading up on that for one of my own cats. Some of the symptoms are eating less and losing weight. They do tend to drink more so perhaps this isn't it. But you could buy some 'Nosorb' (not sure if you have that in your area?) and use that in place of litter and then when he pees, pour it all back into the jar it came in and have the vet test it for diabetes. Not a high tech way to test but could indicate if he has a problem.

Hope things improve soon for Mister Fister.