D.G.
My 3 1/2 y/o daughter has been the same way. She's been potty trained since January, 2011 and she goes through these bouts of accidents. Not sure why this is happening, but I do know the bigger deal I make of it, the worse it is. So.... at least in my case, I just treat it like no big deal, we clean up, and life goes on. She gets back on track on her own within the day. I've heard this is really common. I would NEVER back track to pull-ups or diapers, just have him help clean things up, and be low key about it. I would also give more reminders through the day, sometimes they get so caught up in playing, they just forget or try to hold too long. Good luck, hang in there!