How do you have a mother son talk without a mother son relationship

I had my son at 15. I don't regret him but I also know I was in no place to have a baby so I gave him to my sister. He's always known he's adopted. He calls me "M" and idk if that stands for mom or my name Melanie but I never asked. I've always been in his life. Just in more of a cousin sort of way. My sister passed away last March and my son's dad is... A... Character. He's on a planet that hasnt been discovered yet. That's all I'm gonna say. I picked my son up from school to take him home to his dad. I've been doing it for the past 3 weeks. He has a girlfriend but when I got him I caught him kissing someone who wasn't his girlfriend. When I brought it up he told me what he does with his life is his business. He's 16. A year older than I was when I had him. His mom is gone. His dad is too busy being out of his mind 24/7. He has little guidance. But how do you offer parental guidance without the parental relationship?

Edit: I don't have custody of my son or any rights. I just pick him up from school and hang out with him. His dad has custody of him.

Edit: When I say dad I mean my sister's husband! Probably should have explained that. Not his biological dad, but my sister's husband.