It seems that some rare cats never learn to use a litterbox. My father's cat's kitten was one, and there is an author of a pet training book, who actually had to euthanize her cat due to it incurably urinating on her stove elements.
So your cat used to use the box, and now refuses to? First thing, I agree, is definately get him checked out for health problems. I missed how old it is, but when they age they can have 'accidents'.
My cat is 13 and recently started picking the area behind the couch in the family room as her box (just for peeing)
We finally had to give in and try a second litter pan behind the couch. I know you said you've tried 'personal litterboxes', but did you try something like this - where he's already choosing to go? Is the current litterbox maybe in an area he doesn't like? So far so good for my cat. I think due to her age she does not feel the need to go all the way down the stairs to get to her box, especially her being shy, when the couch offers enough privacy
So we now have a litter pan behind the sofa. What are you gonna do..