Did your vet take a urine sample to check for urinary tract infecitons and crystals in the urine? That is almost always the cause of sudden on set litter box avoidance.
Another cause could be stress. Think about your lives very carefully lately. Something you would not think of as stressful could be causing him stress. New person or pet, or one less person or pet in the household.
New furniture. Unusual activity (more noisy, for example)
Change in litter, change in food, change in laundry detergent.
A strange animal outside in the yard, especially a non neutered male, could cause this.
If a urine culture has not been done, have it done.
Install feliway plug in diffusers, these help both with stress and with territorial issues.
Have at least three litter boxes.
Keep all litter boxes scrupulously
clean. Scoop them all at least three times a day, dump and scrub, putting in all new litter, once a week
I just had a thought...is he peeing outside the litter box, or is he just standing up in the litter box and peeing on the wall?
You can get high sided litter boxes..use plastic tote boxes. I have a female cat who pees standing up, I use these: