K.B.
As hard as it may be, it may just take someone making fun of him to drive the point home. My friend's son went through the same thing at the same age, and they talked to him til they were blue in the face, but it still didn't matter. He wouldn't use the bathroom at school to do #2 and it got to a point that he would have some "leakage", and some kids started teasing him for having a stinky butt. He got upset, but his parents just told him, "Well, that's what happens when you hold it too long and won't go at school!" I would think what happened before would have caused him sufficient embarrassment for him to realize what he needs to do - if not, maybe a talk with the pediatrician is in order. Or maybe at home he is not allowed to leave the bathroom until he makes an attempt to do it himself.