I have always potty trained by letting them run around naked first and once that is mastered I introduce the undies, let them pick it out, my son hates tighty whities, and actually went without underwear for quite a while, until I found the boxer briefs for him, what a difference, I think it was the restriction of the regular boys underwear, almost like a diaper or even a pull up, and then I would just remind him to use the restroom, it worked. Really I think it will come down to just reminding him to use the bathroom, and letting him know that accidents are okay, they happen but maybe next time he could try harder to get to the potty, thats what really worked with my daughter, she liked knowing she was doing good and so long as she tried I was proud of her effort. I know you got a lot of good advice so good luck!