When an accident does occur, spend a few seconds verbally displaying disappointment,'No, your pants are wet, you have to practice some more.' Practice twice going to the pot from the scene of the accident and pulling down pants at a quick pace, then raising them again. Practice another eight times from different locations, so he will remember to rapidly use the bathroom, no matter where he is.
The next step is the inspection. Since he has not changed yet, they are wet. Tell him again that you and his heros are disappointed. Repeat ten times. Have the child take off the wet pants, put them in the laundry, get a new pair, put them on and then clean up any leakage with a sponge.
…