Agreed, it is definitely something only you can decide. I agree with Sarita with weighing how it effects you as well.
I also agree with Beka that sometimes vets are quick (in our minds definitely) to bring up euthanasia in some way, even the most compassionate doctors will mention it as easily as possible, perhaps using words like “might be something to think about”. To play Devil’s Advocate I feel I need to say, that many times when pet owners at the point of even muttering the words “quality of life”, a vet bringing that up as something to consider is a good thing. As someone who has worked at a vet hospital and having seen a huge range of personality types of owners from the callous, to the crazy cat lady… sometimes people come in NEED to hear that word and some even WANT to hear it because they are afraid to use it first.
Sometimes we feel guilt about making that decision, and it helps to have an outsider look at only the clinical part of things. I think if you feel adjusting the environment (ie cutting the side of litter box, more padding for the arthritis) and perhaps more pain meds would help him, that you should talk to your vet about getting more. Maybe if you are preemptive in the conversation, “I know we’re just managing pain right now, but he’s still a happy old bun, and until he tells me he’s ready, I’ll do what I can to make it any easier” (or something similar?), that way the vet knows you don’t need to be reminded about your “options”.
I do not envy you right now.
I did want to say one thing though. Whenever you feel it’s time, it would be a good idea to be sure his bonded buddy is able to sit with him afterwards for closure.