There can be many different reasons for cats to start peeing outside the litterbox. There is two terms used for this and they are horizontal suface and vertical surface. Horizontal means on the floor or anything horizontal, vertical means spraying the walls, furniture, ect...
If your cat is spraying, it could mean he's marking his territory for some reason or he is stressed about something, or more a behavior problem. If he is peeing on the floor, it could mean there is something wrong with him, even though he looks and acts healthy.
Have you changed brands of kitty litter? Changed the location of the litter? Changed something like your furniture around or an airfreshner in your house? These might be a reason cats start to do this.
There is medication for anti-anxiety that works very well to correct this also.
When mine start to pee outside their litter, this always works for me if it's not medical. I confine them to a small space like the bathroom or a bedroom with an immaculately clean litterbox, right after they eat, and for an hour every two hours. So, if he is in the bathroom from 1:00 to 2:00, I put him back in there at 4:00 and so on. When he starts to get better I extend the time out of the bathroom or bedroom little by little. If he gets worse I shorten the time. I also go in and visit him about every 15 minutes while he's in there so he doesnt feel deserted and the behavioral problem doesnt worsen. I never leave the house and leave him unattended either when he's still peeing around the house. It takes a lot of patients to correct this when they start, but it can be corrected! Good luck!