Because there's no way I'm going to ever hold a serious discussion with this kid about why he would do such a thing, I sat down and did some serious thinking about possible reasons.
The only real thing I could come up with is this: he doesn't like using towels because he's ocd and doesn't want to reuse the same thing to dry himself off with.
Granted, this is pure speculation, but I feel like using toilet paper is worse than a towel. The odds of getting poop particles on you seems really high. Plus, eww, it has to get all trapped in his crotch hair and shit.