I've had that happen with Bad @$$ Buddy. He was SO confused about where he belonged, he started peeing on my kitchen floor.

Feliway is what my vet gave me to spray in the air. He gave me the spray rather than the diffuser. I am never without it!!!

Do you have other cats? One of them could be sabbotaging the other while in the box and that's why he's doing it. Did your vet test for crystals? Just because there's no blood in the urine does not necessarily rule out a UTI.

Rule of thumb is one litterbox per cat per household. If you have more than one cat and only one box, that could very well be the problem.