SweetCrusader's list is great. All you can do is be with him until he gets help & keep him safe. Usually an attempt is not made until they are alone, so don't leave him alone. As long as someone it there it is no use to try because help will be called immediately & there is usually nothing they can do about it.

Try to convince him that life is to be lived & to think about what he might miss if it is gone. Those things are irrelevent to someone who is determined to succeed, but it just might trigger a glimmer of hope.

Just remember, you will not be able to stop him in the long run if he is really determined to succeed, but your support can make a difference if the determination is not completely there..