Underwear and Toothbrushes

I'm not sure I will ever get Dylan potty trained. He is by far my most challenging. Yesterday he pooped in his underwear and then, ever so quietly, he went in the bathroom and took them off and put his shorts back on commando. Today we found the underwear, which apparently is more than a one-day process, wet on the sink next to his brother's toothbrush. He had scrubbed his underwear to get them clean. Where does he learn these things? He's never seen me scrubbing his underwear with toothbrushes. This morning, the boys were all sitting at the table ready for breakfast when I had to tell them that I threw all their toothbrushes away and they could pick a new one from the closet this morning. They asked why and I said something I never thought I would need to. "Because your brother was cleaning poop out of his underwear with them." Honestly, where do these ideas come from and can you buy pull-ups for kids going to college because I think Dylan may need it. Ugh.