I'd view this similarly to other questions about interventions.
Similar issues arise if he is becoming addicted to a harmful drug,
or is depressed and suicidal.
I would intervene and use coercion if he were my 10 year old child. If he
were 30 I would probably limit myself to persuasion. 17 is a gray area.
There is a saying in the legal community, "hard cases make bad law".
I'm not sure exactly what it means, but I have always thought of it
as advice that exploring these gray areas is not a fruitful way to
establish fundamental issues. You become embroiled in the specifics of
the situation and it obscures the broad principles.
Hal