One of my best friends is very depressed recently. I'm trying to help him yet he doesn't want to talk about his problems. he's a very secretive and reserved person. he never ever talks about his problem to anymore, not even to his best friends. I'm so worried about him. i really want to help him but it's very hard to approach as he refuses to say a single word of his problems. What should I do?

by the way, the only tiny info i got is that yesterday he asked me what i think of him. i said, "you are a great person and one of the coolest guys i've ever met." He said, "that would be funny if it was true." And i confirmed that it IS true and told him that he needs more self-confidence. Then I presumed that maybe there're some people there think ill of him or look down upon him. So i asked him if it was that but he didn't say anything. Now i'm completely confused. every time i asked him "what's wrong?" or "what happened?" he said "nothing" and i knew there can not be "nothing"! Gosh I SO MUCH want to help him but he keeps hiding his feelings and thoughts which makes me don't know HOW to offer my help. Any suggestion?