Wow, where to start...
I think I should start with seconding the suggestions for professional help. There are amazing people out there who are trained and skilled at helping others work through grief.
Secondly, a few more personal thoughts
- please don't judge him for attempting suicide! It is very popular to throw accusations of selfishness and cowardice at people who survive suicide attempts. The guilt and self doubt that causes does not help the healing process at all. Anyway, it's not true. Suicide is an act of desperation and anyone who doesn't understand it should rather be grateful that they've never experienced that level of despair.
- Consider also getting professional help for your daughter. It must be extremely traumatic to find someone you love like that...
- don't beat yourself up. The signs can be very hard to spot if you don't know what to look for, and most people don't. It's not your fault!
- lastly, on supporting him in his grief, I will say this
* listen. Really listen, without offering advice or encouragement. Just let him talk and show empathy.
* stay away from gliches and platitudes. There may be many fish in the sea, but he doesn't love them and doesn't have dreams for a future with them that was suddenly destroyed. Time will not heal this wound - the best he can hope for is to learn to be happy again, in spite of it.
* Don't even mention pursuing other girls. He will do so when he is ready, and to make him feel pressure to do it earlier, will only result in more pain for him and the girl in question.
* Encourage him to stay busy. The best thing for him now is to get back to his old hobbies and interests. When someone you love dies, it feels impossible that the earth can keep spinning and the people around you can just carry on living. Don't they realise the world has ended? The only way to get out of that hole of disorientation is to get back into life, and to see for yourself that it hasn't ended after all.
Ps. You wil notice that I didn't refer to his young age or the seriousness of the relationship. It is obvious that it was very serious to him, and invalidating his feelings is the last thing he needs right now.
This is all I can think of for now. I hope it helps...