I think Kevin was wearing them just to annoy his mother and once she broke his arm, he finally potty trained. I was also impressed how he lied for her and had such a good convincing story to tell about it so that showed me he must love his mother and he would always point to his broken arm as a way to blackmail her. Of course the mother felt bad she broke his arm and had great remorse for it. She just snapped. It was like she earned his respect when she broke his arm so he decided to potty train as her reward.