I see you feed him dry and occasionally wet. I would try giving him more wet food as cats can be notorious for not drinking enough even if they have the luxury of a water fountain, like your cat does. It's good you have got a vet appointment to check him out. It may be as simple as him being constipated and finding using the litter tray an uncomfortable experience and giving him negative associations with it. If the vet says he is fine then I would definitely get more and larger trays as Claire suggested. Cats can be very fussy with their toilets and it's finding what your particular kitty likes. I know my cat Oscar likes his tray totally clean before he will use it again - he is so high maintenance
Please don't take offence at things people suggest on this forum, we are all trying to help each other and it's kind of like a mass brainstorming session! I know I have taken so much advice from this forum and i'm always learning even though I have had cats in my life for 44 years! Hope the problem sorts itself out very soon.