Guy I'm sleeping with burns himself

It feels weird that I see it and never mention it. There's this guy I kind of casually date. We hook up but also hang out. He's came over and helped me with my college algebra a few times. Overall a very sweet guy. When we first had sex I noticed burn marks in his thighs but I said nothing. Over time I've noticed more and they get worse. IDK if it my place to mention something. How would you handle this situation?