Bit of a different age but when DH was head in the sand about the quality of life of his 1 year old cat who had suspected FIP, someone suggested doing a quality of life questionnaire, so I had us do one separately then compared the results. There were actually places he scored him slightly lower than me, and i think this started to make him think actually AC's quality of life wasn't great. It was hard as before I could see how he was struggling, not playing, not taking an interest in life, not eating with the enthusiasm he used to etc, but I could view him more "objectively" as he was more DH cat so he obviously had emotions mixed with it. It lead him to agree that if the ultrasound all but confirmed it was FIP, to pts, as the end stages of FIP are horrible
It actually ended before AC's ultrasound, during the day he suddenly peed in a box a delivery had arrived in. Despite his FIP caused diarrhea he'd always been very good about going in his tray. Started to think "hmmmm". Was then woken at 3am that night by him having explosive diarrhea on the bedroom carpet, to me that was the red line crossed, his dignity had gone. It was also like he wasn't there, he seemed completely oblivious. He was pts that morning, having been shut in the kitchen for the rest of the night in case of more accidents. On going down to him that morning, he was completely zoned out, dare I say ready?
So what I'm trying to say is it's worth getting you and OH to do a weekly quality of life questionnaire, you can Google them and they're available online. Keep a track of the scores you give to see how cat is doing - it may be you get to a point where the "bad" outweighs the "good" and that will probably be when a discussion needs to be had, but you almost need to let OH realise this without it being down to you saying so, I had to with AC as he was mainly OH's.
For me toileting outside the tray counts as dignity having gone for the poor cat, and that's when decisions need to be made
Our dear little girl I'd had all through my teenage years and into my early 20s when I lived at home started to lose her back end. Some "reasonable adjustments" we made for her were to make sure her sleeping spots were easily accessible, we put blankets over a gap between the bed and the windowsill she liked to go on, after her legs caused her to tip down into the gap, this way she now had a bridge. Putting steps/a step by places she liked to jump up to, so she had to "push" less. Is his tray easy for him to get into? Could you use one that's very shallow as pp have suggested so he doesn't have to use his back legs as much and can just walk straight in and out without having to "step" it may be it makes him a bit nervous if he's having to balance and his legs are iffy. Could you have a couple of trays? Pita for you, but if he's wobbly and wakes up on one floor, he may not feel he can make it to the tray if he has to travel to find it. It may even be neighbour didn't really do much with regards to his toileting so he was having accidents there
I'd give his "reasonable adjustments" a go, if he's still struggling Despite them it may be OH realises that actually, however hard a decision it is (and it is heartbreaking) that it's "time".
Has he had blood tests? Our little girl mentioned above had kidney disease and was on medication for it and towards the end, she was starting to get constipation, which caused her to have a couple of accidents, as she'd think she'd been then a little bit later the rest would come out, when she was sleeping. After a night when she was in visible distress due to struggling in tne tray my mum made "that" decision even though she was still purring and chatting to us
It's awful to go through with a much loved cat, however it has always stuck in my head what my mum said when we had childhood pets pts "at least we can do this for animals, humans you have to watch suffer"
Certainly for your guy, if he's happy in himself, I'd try the different adjustments for him to help him get around easier, but also start the quality of life questionnaires, it'll help see if the changes are helping him as well