Part of the best answer includes IF you have raised him hearing about God and His Word, since you are wanting to know when to "let God"... Rather than telling and warning him from your own words... remind him of what God says, and how God loves him and when he's ready, God will help him. Just keep reminding him so that when he does hit bottom, he'll look to God. Prayerfully he will reach this place sooner rather than at the end of a long miserable life.

We cannot change others. Worrying does not change a single thing EXCEPT it changes us, our attitude, zaps our time and energy...

So... the idea of "letting go and letting God" comes from knowing and acknowledging those facts: we can't change anyone, worrying does no good. THEN comes the "God, I can do nothing but give my love and support to my son and be firm for his own good (called tough love)... I put it into YOUR hands... I put His safety and life into YOUR hands.... I will trust You. Once you do that---and this is at least a daily reaffirmation every time the worry begins too---you can put your own life back together and be a good example for your son to rely upon.