I can understand you worrying about your son. Your radar is picking up the subtle signs that something is not right. Unfortunately there is not much you can do. Be honest with him and set clear boundaries.
Because he is an adult and will make his own choices. The more you try to influence him, the more he will distance himself from you. All you can do is share your feelings with him. Tell him you feel he is more distant, tell him you worry about him and his use, tell him about the family history. But don't over do it or his will stay far away from you. You need to maintain a relationship so when he is ready for help he will turn to you. This could be a difficult road as he may have to experience some negative consequences from his use. Get support for yourself, go to Alanon and/or counseling.
Set boundaries with him. It sounds like he lives with you, maybe you need to set a date for him to move out after graduation. It is not helpful for you to know too much about his life. It is not helpful for him to not be responsible for himself. He needs to grow up and manage his life. He is going to make mistakes, you are going to need to distance yourself from these mistakes. It is hard to maintain the delicate balance between maintaining a relationship and detaching from his problems/mistakes. That is where Alanon or counseling will be very valuable for you. You need to maintain your well being. Good luck to you, let me know if you have further questions.