Artistic needs as a reason for substance use

This is actually a common reason people have and or give to use substances. I am unsure that I can appropriately tell you how to help him connect his use to the negative outcomes in his life.

The technique of Motivational Interviewing is the best way to do that and it does take training and experience.

If he enters into a dialogue with you about it, you may simply ask him if he connects his use with any stuff in his life that he is not completely happy with. Not engaging his defensiveness and need to protect his use is the best way to at least get him to start thinking about it and talking about it. Talk to him about his goals for the future in a way that does not make him feel judged. It is his life. If your parents aren't happy with him living there, it is there place to tell him. Be honest with him if he is open to your opinion.