Hm. Well, I would watch him like a hawk when he's near it. I assume he is marking it, not actually pottying? If he's pottying, maybe you can get him on a schedule and make sure he's always empty. If he is marking, wait for him to lift his leg, and clap your hands, or otherwise indicate your displeasure. Then take him outside and reward him for his potty.
Also, make sure to clean up the accidents with an enzyme cleaner so he's not drawn back to the scene of the crime.
Hope this helps.
