My first thought would be that he could have a UTI and you should make an appointment with your vet to have him checked out. Cats that have this find it very painful to pee and they will pee in inappropriate places.

If he checks out fine then it must be behavioral. Maybe a animal in the past has peed there and he now smells the scent or maybe he's been stressed out lately. Maybe another cat isn't allowing him to use the litter box. There could be many things going on that you're unaware of.

Do you have any feliway spray or plugins? If not I'd buy some of these but you don't want to spray or use it near the litter box. It'll help calm him down and hopefully this will stop. You also need to clean the baseboards with an enzyme cleaner to get rid of the urine smell so none of your animals will keep peeing there. I've also used Cat Attract Litter by Dr. Elsey which you can buy at Petsmart or online. It attracts cats to use the litter and it works immediately. Yes, it does cost more but it helped solve my problem several years ago. Now I'm able to use Dr. Elsey's Precious Cat Ultra Clumping Litter which is unscented and it clumps very well. Here's a great link that will hopefully answer any of your questions: http://www.catinfo.org/litterbox.htm. Good luck.