I went through this around 18. I hide my emotions (like most guys), so my parents had no clue. I felt hopeless, that nothing in life was going to work out, tired of hurting. I didn't see a purpose to living; didn't know what I was going to do with myself for the rest of my life. You lose perspective on the joys in life. You feel like life is all pain and it's never going to end, and it'd simply be better to not exist anymore than to wake up every day dealing with all that.
Eventually, I tried to kill myself. I went the overdose route - we didn't have a gun in the house, and I didn't want to leave a crime scene for my family anyway. Right after I did it, I called a friend. Not entirely sure why. A desperate last minute plea for attention? Fortunately for me, my friend was savvy to what was going on, and long story short, they and my parents came to the house and I was rushed to the hospital.
The best thing that came from this is that it finally 1) alerted my parents to the pain I was going through and 2) gave me an opportunity to talk to them about it. I didn't need answers. They didn't help me "solve" anything. But it reminded me how much I was loved, and the catharsis in simply knowing that my parents were aware of the hurt I felt helped me get through it.
That was 12 years ago. I'm 30 now. Married for 7 years. Great friends, better family. No kids, because my wife and I have decided to keep our lives simple so that we can volunteer our time. It's a sacrifice, but it's a rewarding one.
I look back on that time and it was truly a turning point. It's hard to fathom now. I don't think about it much anymore, but when I do, I feel a bit of shame - I almost cost myself an opportunity to live a pretty wonderful life. That hopelessness I felt seems so short-sighted now. I cope with humor, so I like to joke to myself that I was such a failure back then that I couldn't even kill myself correctly. But it was the best failure I ever had. The days following that attempt were the springboard for the rest of my life. But it wasn't because the suicide attempt was cathartic, or put things into perspective. That's crap. That attempt was the biggest mistake of my life and the one single regret I have. It's the only thing in my life that I could "undo" if I could. But, it's the event that finally revealed to my parents what I was feeling and allowed them to be there for me. And that's what turned my life around; that's what saved me.
I was fortunate. I tried to throw away my life, but I survived and went on to realize what a mistake it was. Other people aren't so fortunate. My point is, talk to your son. Let him know you care about him. Don't try to fix his problems, because you can't. And don't minimize his problems, because they're big to him. But let him know you care, that you're sorry he's hurting, that you love him, that you're so grateful to have him, and that you're here for anything he needs. There's not much in the world that's more powerful than a parent's love.