Develop a sense of urgency about this. Put him in pull ups, but insist he have his movement in the bathroom. Take him into where it should happen. When finished he should be part of the clean up. Flush, get a new pull up--not back into underwear--then sit him on the toilet for a few seconds. continue to encourage potty sitting. Develop the reward system. If he moves outside the bathroom take him in for clean up and remind him this is where it happens. If his school will not take someone in diapers, remind him why he can't go to school. Be honest. Peer pressure, visual lessons, and not letting him get "comfortable" with BM's elsewhere usually works. Good luck.
Wayne Yankus, MD, FAAP
expert panelist: pediatrics