If a cat begins to have some stirrings, it can quickly become a learned behavior. Our noses aren't a good judge of what a cat smells, so clean and clean again with an enzyme cleaner/anti icky poo. There are other products out there, but nothing works as well as anti icky poo.
I always feel best having two available even for a single cat. One for wizzing, one for dropping deuces. As Witchy stated, pine is a big ask for a cat that has inappropriate elimination issues. Dr Elsey's Cat Attract is our go to, unscented and when we were having issues with one of our DSH boys, we had him quarantined with 4 or 5 litters. What we came to learn was he wouldn't soil a standard size box more than once and even large boxes, such as big sterilite containers, he wouldn't pee and poop in the same box. Final resolve for our big boy Kebu, was to have a remote box for pooping and 5 other boxes for 3 cats and 2x a day cleaning.
Oh, and when your boy does use a litter box, can you describe his behavior?