Be very concerned for his safety and well being. A student succesfully committed suicide there by hanging herself. If he is suicidal, he belongs in a psychiatric setting, not a 'behavior modification' camp.
Because he is 17 1/2, he can wait out his time until he turns 18, which he will most likely do. He is at a lower risk of brainwashing than the younger students, because he has a 'hope' to look forward to due to his age. He will not receive any help at SCL, only discipline, humiliation, and quite possibly physical and emotional abuse. He sounds like the type that might spend time up in the hobbit, and in worksheets, for not complying with the program.
When he turns 18 and wishes to leave the program is when you can REALLY help. The program (via his parents) will offer an exit plan ranging from nothing but the clothes on his back, to possibly some cash and a ride to the closest town. Either way, this is when you can really help him. Otherwise, he will be pressured by scare tactics to stay in the 18 year old program. It is scary to face walking down the street into homelessness at 18!
Somehow, you need to get letters into him. This will only usually happen if his parents allow this. The best way is to slip in your letters with their letters, because they write a code on the letter or something, or put a sticker on it, I'm not positive. In that letter instruct him to keep quiet about you helping him after he leaves to prevent any actions against you. Give him your phone number and tell him you will wait for his call on his 18th birthday, and you will wire him money and a train ticket, etc. This will give him immense hope, and he will certainly feel better about the future. This is one idea at least. At the very least, it might keep him from hurting himself or running.
You say he escaped his transport, went up on a bridge and threatened suicide, so he should have been placed under a 72 hour watch at a psychiatric facility. Not shipped to a private teen prison with the occasional large group brainwash session to boot. It's crazy his parents believe this place is the best for him... utterly insane.
It reminds me very much of my own situation. Unfortunately, he is in for a shitty experience, especially because he sounds like the type which will reject the program's flawed ideology. At least he will remain 'himself', but he will get some major shit for 'not working the program' and such. He can just wait his time out, but the damage done to him, his family, and everyone who knew him will already be done. This is a fucked up situation for all involved... accept WWASP, they are making out just fine.
Good luck.