I agree with SCraver - and I had the same issue and took my cat to a vet and he did find a health issue with him and told me you always want one more litter box than number of cats (now we have two cats and three boxes) and it's fine. They DO pee in one and poop in the other but they're all in different areas so depending on how safe they feel in each spot, I guess. Make sure it's not near something loud - like near a dryer that buzzes and scares him, or where he can be backed into a corner if you have another cat. GOOD LUCK and be patient. Sometimes it takes a while for them to get back to it. If it's a hooded litter box, maybe buy another one to give him an option. My neighbor had that issue - then realized her cat, who'd used the hooded box for ages, had probably been cornered in it by the other cat, and now didn't want to use it. But he uses the open air one - go figure
I used Urine Gone - you spray it on the spot and let it soak into the pad and don't pat it dry - just let it dry over the next day and they're not drawn back to that spot.