He needs a schedule where he forces himself to use the restroom every few hours. Even if he doesn't think he needs to go, he has to force himself to try. If he gets constipated add more greens to his diet. He might protest the vegetables, but they help with constipation.

I tend to get the "Must find a bathroom NOW" urge and then only have 30sec to a 1 min to get to one in time. Scheduling regular restroom breaks helped lower how often these kind of potential accidents were happening. Basically regularly relieving the pressure on my bladder gave me more control.