You can't logic and reason your way out of this kind of depression. One conversation is not enough to convince someone that the huge wall they have built around themselves needs to come down. They might not even realize they've been building that wall.
Fair enough. What do you suggest one starts with, then? What are the points you'd bring up to make the guy see that the world is not a terrible place (so that those would cook in his head in the meantime)? How do you start bringing down the wall from your side? Can you, even?
There's nothing I can say to make someone change their point of view. It has to come from that person. All I can say is that he's wrong. The things he thinks aren't true. If you want to take that wall down, you have to examine each brick. Why did you put that brick there? Because of an experience or moment - you have to look back on it and either accept it and move on, or totally change your point of view on the experience. It's not my wall, so I can't tell you what the bricks are made of. You have to do that yourself, with a lot of introspection. The problem with someone who's depressed is, that you can't think clearly or logically about a moment, and come to a different conclusion. You are already in the depths of an emotional state where you can't change your mind without help (help from a professional therapist or help from an anti-depressant).