J.
I would actually reverse your method and simply praise him beyond belief when he DOES go potty on his own. He may be trying to get any attention at all, even if it's negative, by having these "accidents". Try not punishing and just praising, and see if it gets you anywhere, since the punishing alone doesn't seem to be working. Good luck--he will also get better as he gets older. My four year-old still has accidents now and then, even though he tries not to. They just don't quite have the muscle memory yet to do it right every time.