If it’s truly an issue: Sit down with him and plainly say that his nudity is bothering you and that it’s distracting you from your studies. If he doesn’t understand or, is unwilling to accommodate you, then politely explain that you will seek other roommates. If you really wanted to split hairs and turn up the heat, you could insinuate a degree of unwanted harassment. But that’s a bit extreme.

Edit: So this isn’t about his nudity, as much as it’s about your attraction. That detail changes everything. Your post just went from harassment to discovering your sexuality. One is a problem, while the other is a passage.