Thank you everyone! A lot of these suggestions are great, and are things we have done and do do, usually when I have both boys on a weekend, but then they usually hang with each other and kind of ignore me, which I think is pretty typical for teens.
Where I feel like I am struggling is more with the weekdays. Our weekdays this fall will be pretty busy with sports, and homework, and chores that need to be done.
I feel like with my own kid, we have a lot of little ways we connect throughout an evening. So, we might listen to an old time radio show we both enjoy in the car, and chat about it, or he likes to cook so he might help me in the kitchen for a little bit, or he might share something interesting he learned at school, or we might play a quick hand of cards. It might be 15 minutes of connection, once or twice a day, and then he's back to doing homework, or I've dropped him at school or sports practice, or he's playing a video game.
My nephew and I don't have those moments, and the result is that I feel like all of our interactions consist of me nagging or directing him to do something he'd rather not do. Not that I'm spending an enormous amount of time nagging, but because I'm not really spending a lot of time interacting with him at all, there's no positive to outweigh the nagging, the way there is for my own kid, who requires nagging too!