How to get a teen to wear their seatbelt while driving?

I’m freaking out! My nephew is in college, he’s super smart, & is a star athlete; however, he’s not being smart while he is driving! His parents just brought him a new car for his birthday & my whole family has already talked to him multiple times about wearing a seatbelt and just regular safety precautions while driving. Well today he came to visit, & my brother said he pulled up with no seatbelt on!!! Im going to talk to my brother about how much it’s concerning me, & Im definitely starting to feel like he shouldn’t have a car at all. Idk what to do, because I’m not his parent & I know he’s not going to listen to me. Its scaring me. I’ve known so many people who died when I was in high school just from making stupid decisions while driving, I don’t want that to happen ever!! Any advice on how I should approach this situation?