D.B.
Given the way it started, I would say just ignore it and he should stop soon on his own. I know it's probably irritating to have to wash underwear so often, especially when they're barely dirty. Hopefully it won't last much longer! My friend's daughter had a stomach virus when she was 4 so her mom gave her a cup to carry in case she couldn't make it to the bathroom in time. She carried the cup with her for a month! But she did finally move on. Hang in there!