My cat has a litter box, but hates to use it. Really. He prefers to use the soil in my banana tree, and I think he's done a good job in killing it. (Plus the fungus, and the spidermites) I eventually had to put a big piece of bristol board over the soil so he couldn't get in it anymore. Now, he uses the litter box.
There are sprays you can buy that apparently are offensive to cat's, and it keeps them away from whatever you spray. They also hate citrus scents. It's kind of strange that the cat is finding different places to do its business all the time, as usually they stick to one area.
Be patient, cats don't know. Certainly he's not doing it to piss you off.
You could even ask a vet? I'm sure they've got some good suggestions.