I agree with mokie - the best thing for you to do right now is educate yourself. And make sure you take care of your own needs too. I would not continue to send him letters. At this point, it sounds like it will only agitate things further and might be viewed as harassment by his parents.

As for how long to wait? That's not something we can really answer. My sense is that if it's meant to be, he will contact you once he's out of this crisis. But right now I think the best thing to do is educate yourself and give him the space to heal.