Hi L.,
FYI, I am a veterinary technician. First bring him to your vet and get a urine sample checked for an infection. Next, go to the pet store and get some kind of enzymatic cleaner to use on any areas he has peed on. Even if you can't smell anything, he may still be able to. And even if you clear up an infection, he may still go to the same spots because it already smells like urine to him.
If it is not an infection, then it is behavioral, which is much more difficult to deal with. If this is the case, try a different type of cat litter, try a different location for the litterbox...if you have other cats, maybe you need another litterbox (sometimes kitties are picky about sharing their box). Keep an eye out for neighborhood cats hanging out in your yard...sometimes that will cause them to start marking their territory.
I'm sure your vet can give you some ideas too...
Good Luck!
A.