Shift responsibility from your shoulders to his.

 

Do not let him think that you feel responsible if he behaves badly.  Remind him that he is good for his own future.  You are just there to help him. Do not let him take you hostage by threatening to quit being a good student (because he thinks you care about that very much).  "I need you to be a good person first (so don't threaten me), and a good student."

Have him do chores.  Instill in him that YOU need his help.  Your old self will thank you for that.