by the time WE find out the situation is often far worse than we are capable of imagining, ie it's gone on longer and the intake is higher, and/or they've done some rather despicable things. not always, of course.
understand that your first "talk" might not go so well.....he is likely to be hostile and defensive and may just shut down entirely. OR he may concede a bit of behavior, but downplay it. the best you can do is express concern and suggest that he change his course SOONER rather than LATER. then it's up to him.....