Not to offend anyone or start any arguements or negative comments. Just stating a fact: Persians are notorious for having litter box issues. This is the number one reason many of them are turned into shelters or given up.
Phoebe is such a Persian. I don't know your whole situation but I will give you some things and ideas that worked for me and for Phoebe. #1: Get a 2nd litterbox. The rule of thumb is one per cat but I have 2 per cat because I have a couple of picky ones that do this... They poo in one and pee in another. It's wierd but they like a separate box for each "deed." #2: Clean it asap! Phoebe was very picky and if she had already gone in one and I hadn't cleaned it fast enough, she would find a sink or a plant (she's the reason I don't keep plants in the house any longer!) or a corner to go in.
#3: Corners that he goes in a lot, put the boxes there. It could be the location. He goes to that spot because it smells like it now, no matter how well you have cleaned. Putting a box there will help with less messes. I know it's not very asthetic to have litterboxes all over the house but either that or put some artistic cat condos or trees there so he knows it's his area but not to go there. Cats don't go where they sleep or play. #4: Use enzyme cleaners like Nature's Miracle to clean the messes. DON'T FORGET THE WALLS! If he can smell it, and he can with just regular cleaner and even if you can't, he will keep going. Also invest in the calming collar. It helped Phoebe. She never responded to Feliway. But the calming collar helped her tremendously. It's cheaper and works automatically, unlike Feliway in a lot of cases.
However, first thing is first, get him to a vet to rule out medical issues.