Kick him out. Having my parents cut off financial support to me was the kick in the ass I needed to get into shape. Best thing that ever happened to me.
You talk to him. Drag him out to sit in the yard and talk for an hour. Demand he ride with you to the grocery store.
You listen to his reality. You ask him about what games he is playing and how they work. You ask him about what he wishes for more of. You respect him and love him and build trust.
You tell him hard truths, gently, when he needs to hear them.
You accept that he will need to wander in the desert to find his own truth, and might end up somewhere other than you.
You reach back out when you miss him.
You expect him to know things you don’t, and you take joy in discovering new things about him you didn’t know were there.
You tell him when he is hurting you, draw the line on abusive behaviors. Explain why his behaviors are wrong and describe what you need from him to feel great about the relationship.
You tell him about your own struggles and weaknesses. You talk about your hopes for improvement. You are self deprecating.
You laugh at his jokes. You are playful. You try to make him laugh even if he doesn’t appreciate your humor.
You treat him the same way you treat the adults that you love and respect most, so that he grows into one.
Tough love is sometimes the best form of love. Be prepared to be resented; be prepared for the child to cut you out of their life.
Accept it's their choice. You're probably giving them more choice than you ever have before. And it's scary for both of you. But it's literally the only way that some people can learn how to grow up.