At the end of the day, somehow this young man derailed. It's easy to blame the parent(s) (and I've done it myself), especially when it is clear s/he/they have been less than attentive. But some kids/people are just problems for whatever reason. In any event, it's moot now with THIS child. Unless there is a sibling involved, the hindsighting, soul-searching and angsting is meaningless.
HOWEVER, this does not mean that this 17yo will be eligible for emancupation - constructive or otherwise. He is living with a parent, by agreement of both parents. As such, he should be supported by both parents. "Fair" or not. A bitter pill to swallow, but there it is.