I think you're not a real parent and are trolling for flames. However, I'll answer the question for argument's sake.
Send him to a 30 day detox program -- one that also serves adults, if you can find one. And I mean twenty and thirty something adults. They may separate adolescents from the twenty and thirty somethings, but it should be substantially the same procedures and services for both groups.
If that doesn't work, send him to a different 30 day detox program. You have three basic kinds: education based, 12 step, and aversion.
Aversion is usually a very short thing--like less than a week.
If one kind doesn't work for him, try the next kind. Keep going with 30 day or so rehabs, bring him home, see if it "took", send him back if necessary, etc., until one of them takes.
Don't go for anything over 100 days for drug abuse, not under any circumstances, and make sure you try one of each kind of the 30 day or shorter rehabs first.
Resign yourself to him having to go back and make up his education, in the form of a GED and remedial courses at the local junior college, after he grows up and matures enough to want it.
Once a kid gets to 16 or so, you really can't make them learn. They have to choose it for themselves before they can get an education.
Now, about the suicide threats and apparent depression: That's the most serious problem the kid has. That's the one that could kill him.
Either the kid has a biological mental health problem that is causing the drug abuse and school failure, or the drug abuse is causing the depressed feelings and the suicide threats. Drugs like meth, and xanax, can have a very hard "down" when someone comes off them. They can make a user feel depressed or suicidal even when there is no underlying mental illness.
So first you get him through detox so his system is clean and the drugs aren't bouncing his mood all over the place, and then when he's towards the end of the rehab when you know for sure he's not taking illicit drugs, have a good psychiatrist examine him and see if he's going through an episode of depression for some reason or has a mood disorder (major depression, bipolar disorder), or a personality disorder, or whatever.
If it turns out that he has a major mental illness, chances are that's causing all his other problems. Take care of that, and it becomes a lot easier to address the other stuff. Don't take care of that, and you don't have a prayer with the other stuff.
For a major mental illness, if they can't get him stabilized within the three weeks or so a regular mental hospital would be willing to try, and if he's still actively a danger to himself or others, then and only then you might have to put him in a reputable, medical model RTC while they try him on different meds and different combinations of meds until they can get him stabilized.
If he's mentally ill and they can get him stabilized at least enough not to be dangerous, then you need to consult with the psychiatrist and use the least restrictive care setting in which the psychiatrist says he can appropriately be treated.
People with mood disorders can change constantly, so the least restrictive setting today may be too restrictive or not restrictive enough tomorrow.
If it turns out that he's mentally ill, get in touch with NAMI or CABF--they have resources that can help you educate yourself and learn to cope with caring for a mentally ill family member.
Julie
(I have bipolar disorder, a child with bipolar disorder, and a teenage foster daughter with PTSD--so I have a lot of experience coping both as a patient and as a parent.)