Two of my cats happily use their litter trays and never go anywhere else. However, my blind cat hates weeing in the trays. Once they find somewhere else, it is really difficult to stop them using that place again. It's a real problem. I have tried spraying him with a plant spray when he's about to perform, which deters him at that time and then he goes back again. Try putting a litter tray in the place he is using. That can often work. Then you can slowly move it to where you want him to go. I now put my cat outside on the shingle when I get up in the morning, take him out again during the day on several occasions and then last thing at night. Believe it or not, he now wees to order and then I take him inside again! Better than puddles indoors!