Our two year old son is almost fully potty trained. He will use the potty for both pee and poop, and he has even taken a few naps (when we forgot to put on a diaper) and woken up dry. However, he is refusing to wear underwear. Currently we let him run around bottomless, because it's just him, my wife and myself living in our house. We want him to start wearing underwear, but he will fight it pretty intensely or the few times he's actually worn them will pee in them.
What are some methods my wife and I can use to teach him to wear underwear and to not potty in them?