I agree with Ang. You may want to put more attention on what you want, rather than what you don't want. Also, as you are being non-reactive to his "accidents," have him do as much of the clean up as he can, with your supervision.

Experiencing the natural consequences of him having more work to do than he would if he just went to the bathroom may turn this behavior around pretty quick.

check out some free behavior support http://focusedgood.blogspot.com/