He (and you) needs to take the long view:
Accept and work through any consequences and penalties with grace, hard as it might be.
See this as a wake up call.
Work on improving and redeeming himself, his choices, his behaviour. Maybe he could think about the people he's mixing with.
Attend self improvement things like rehab.
He needs to look at the future, get good things onto his CV like volunteering, work experience.
Get references secured. Work on earning their trust and support. A character reference from people of note.
And I think you need to strike the right balance - being supportive and there for him, as he does this, getting past your own disappointment and anxiety, putting aside any urge to do it for him.
And keep an eye on the 17 year old, make sure he's doing ok.