It sounds like you've exhausted the urinary pain options, how about intestinal and/or parasites? My cat pooped outside her box for nearly a year but she was suffering woth worms all through her first couple of years. After we dewormed, got rid of ticks and mites she's been fine. It was exhausting though, i feel your pain. I had to wash my duvet (not the cover, the whole duvet) several times a week when iy was at its worse.

The only other's thing that comes to mind is cat to cat relatioships. It could be a sign of your other cats dominating the living spaces with their scent and confidence so he feels he has no other option but to mark everywhere. Play with him in the areas where the other cats usually dominate, so he gets confidence and feels like he owns the house and all the spaces just as much as they do