Hi mollyandstan, welcome to pets.ca.
What types of litters have you tried (brand, type, scented or unscented, etc)? How much litter do you put in the box? (couple inches? more? less?)
I know you say you keep the box clean but different people have different ideas of what "clean" is, so just to confirm, how often do the boxes get scooped?
How much does he (Stan?) weigh? Perhaps he needs an even bigger box, like one of those under-the-bed storage boxes. I find even the really big litter boxes sold at most pet stores are still woefully small.
Couple things to try if you haven't already:
Dr. Elsey's Cat Attract
litter works great at convincing most cats to use the litter box again.
spray and room diffusers. Plug the diffusers in a room or two that he uses frequently (not where the litter boxes are though), and use the spray directly on the areas that he's peeing. Which reminds me, what are you cleaning the urine spots with?
And lastly, what do your kitties eat? Diet plays a huge role in urinary tract health and even though the vet said he doesn't have anything medically wrong (what tests were done, do you know?), he could still have chronic low-level cystitis that makes him feel uncomfortable and causes him to avoid litter boxes. Wet food can help with that.
Some links to check out: