We are sure out 20 year old is experiencing some dementia which she shows in Deaf Cat Howl. She will walk by us in the living room, get up on the chesterfield, face the back and HOOWWWWWLLLLLLLLLLLLLLLLLLLL. We will yell at her (because she's rather deaf) GINGER and she'll look around in surprise as if to say, "Oh, there you are" and lie down and go to sleep. I do feel it's definitely complicated by her hearing loss but she shows other signs as well.
For the litter box problems, do you think that could be partly arthritis? Lifting legs over the side might just be painful so he doesn't do it? I wonder if this is even more the case if he is still bathrooming in the right place, just not right in the box? We think this might be the case with our 18 year old. She poops in the box but pees outside, beside it, on papers. The other way around would be easier to clean up but we don't see any signs that she is mentally losing it so we wonder about the arthritis, she definitely has it.
It's tough when they age poorly. We are due for annuals for all the critters so I'll be asking about several things for the two oldest cats.
Oh, I am giving Ginger Cartrophen Vet injections for her arthritis. It really helps. I may have to go to once a week instead of every second week but it's not too expensive. My Vet pre-loads syringes for me and I pay about $27 with tax for 5. Just in case you think arthritis may be a factor in your litter box problems.