Have you tried putting a bit of food in each of the corners he's going in? Cats hate to go where they eat, and if it's becoming a habit that he's going there, that'll help to break it. Also, is there something about the litter box that's bothering him? Is it next to something noisy, or is it hard to get to in any way? Is it next to something smelly? (by smelly, I mean smelly for a cat, like perfume or cleaning products)