How can i stop my cat from spraying?

My indoor only male cat (Rajah) began spraying in Jan. He's 4yrs old and I had him for 2yrs now (prior to that he was an outdoor cat) He was neutered at age 2. He adjusted quite well to being an indoor cat, began using the litter box right away, did not scratch furniture, and did not spray at all for almost 2 yrs. In Jan. of this yr, noticed Rajah had begun to spray on the walls and sometimes peed on the carpet. Took him to the vet, turned out he had a urinary tract infection. The vet gave him some medication and sent him home, uring us the spraying & urinating should stop when the infection ped. 2 wks later the peeing on the carpet had stopped but the spraying had not so we took him back to the vet to make sure the medicine had worked. He checked out perfectly no signs of infection or any other issues. Vet suggested we add an additional litterbox. Extra litterbox did not solve the issue. Next came the feliway plug in diffuser (no success) and the feliway collar (no success). Called vet for help again, she suggested crating him for about two weeks w/ only room for his litterbox, food and water. At the end of the two weeks it seemed he had gotten the idea, as he hadn't sprayed in the crate, let him out in the apt. where he was fine for a couple of days but now the habit has returned! I'm at my wits end, and need advice on what else I can do! As for details of the cleaning process, I clean the litter boxes every day and completely replace the litter every 2 weeks. When he sprays it is only on the wall (which makes it hard to clean) we use a product called Get Serious Extractor which is supposed to remove the odors, when this didn't work we began to follow the step with a 50/50 mix of water and vinegar which also hasn't deterred him. What else is left to do!!!???!!