I have prevented people from suicide three times, first when I was 15 years old too. Not that it makes me an expert on this, but maybe I can tell you some useful things from my little experience.
Well, no doubts, it's extremely heavy trying to prevent someone from suicide. You might get hopeless of it at first, since it takes some time before they actually realize what you're saying. But that shouldn't make you stop helping, because if you stop, you can be sure that it goes wrong. I know it's hard, but if you go for it, it's worth it.
First you probably try to give him reasons to live for. That could help, but most likely won't. The thing with suicidal people is, that they see a problem, or a pile of problems, and exaggerate them. Let it be clear: it's totally understandable that they do that. But it's not the right thing. According to what I read somewhere else on this site: they're focussing on the problem rather than the right solution. And they probably won't change that attitude as long as they're suicidal. So what you need to do, is asking him questions. No random questions of course, ask him deep questions. Let him think about himself, in order to get to know the actual core of his problem(s). If you reached that point, try to reduce his exaggerated view on his problems. Not by tackling the problem itself, but just by asking him questions again, about the seriousness of his problem, about how 'little' it actually is. Explain him what his problems are, and try to understand them. Reducing the 'size' of his problem (from his eyes) simultaneously reduces the reasons he has for taking his life. And during this process, be clear that he absolutely should not commit suicide as long as you're talking to him. And another thing: be there for him in the first place. Not that I think you aren't there for him, but his life has to be the most important to you as long as he's suicidal. Whether your parents call you to eat: keep talking to him instead of eating. Whether you have to do some important homework: keep talking to him instead of doing homework. Whether it's 3 am: keep talking to him instead of going to sleep. Just sacrifice anything in order to save a life.
And pay attention to what you say or ask him. A wrong word could be fatal. On the other hand: as long as you keep asking questions, he will answer them. You should not really be afraid of saying something wrong, just keep asking questions that lead you to the core of his problems. If you let him think about himself, he probably will feel that your questions are somewhat helping.
But if you notice that it's starting to annoy him, then just stop talking for a while, to make sure he won't stop talking to you forever.
I have tried to explain you the things I learned from my little experience. Of course all cases are individual cases, so I just hope that this is applicable to your friend's case. But never get desperate, because there's definitely a way out. As I said, this is going to be a very heavy process, but it's worth it. After you prevented him for suicide, he'll most likely be endlessly grateful to you. So it won't be for nothing! And I don't know if you're religious, but what helped me too, is praying. And I believe that God helped me prevent those people from suicide. But if you aren't religious, I can't force you to be anyways.
I wish you strength and courage to fulfill this hard time, and to help your friend through it. Be careful with practicing my tips; I will feel guilty for life if one of my tips led him to committing suicide. You know what you can do and what you cannot do to him. Just take my tips into consideration and decide whether they're useful for you or not. I hope they help. Take care my dear!