I'm sorry that you're having this problem. How many litterboxes do you have? If you only have 1 or 2 then I'd get a couple more. I'd also buy some cat attract litter. You can buy it online or at Petsmart.

When I was having peeing problems, I added 3 more litterboxes and then added cat attract litter on top of my old litter. It worked right away. I've also been using Feliway plugins and spray and this also seems to make the cats more at ease. I'd also take him to the vet to rule out any health issues. Good luck.