Try talking to the guy. Tell him there's stuff to live for, etc, etc. Through googling I found:
Florida Suicide & Crisis Hotlines - When You Feel You Can't Go On... Call a Suicide Hotline. / SuicideHotlines.com - Direction for immediate crisis intervention for the gravely suicidal & treatment for major clinical suicidal depression.
Might not be real, I would check it out if I were you.

First thing to do is get a chat log of him saying this. The next would be to contact the Miami police. If you want to help them, get the admin for that Vent server and ask him for help. Try and see if he can get the kid's IP address, send that to Miami police. They will then contact the kid's ISP(probably need a subpoena for this), and attempt to contact him.

No guarantees, as Miami is a big city. You might also not be taken seriously, either.