He's going to pick a 'safe' area, and he's going to use the same spot all the time. Best to put a litter box in the place he's decided to go. If you want to trying moving it, keep the box a little dirty but clean the area where he was going before with vinegar, and move the box, although not too far at first. And I'm not sure that he'll want to go out in the open, but you can try a second box there with a little dirty litter from the first box in it.
Good luck!
sas :bunnydance: