Sounds like he's afraid of premature rejection. I'd mark him in the super shy column. With shy people it is not about how approachable you make yourself seem. There is this security barrier with all people and bringing that down even for someone who seems approachable is difficult and terrifying. If you like him you may need to take the initiative and bluntly ask him out. You can't hint at it by trying to coax him into conversations. He's second guessing every word he speaks right now.

And the attitudes of many of the other posters here reinforce that fear. A man being shy is immature or is not into you or is not worth the effort to pursue? To hell with that toxic extroversion noise. I've been that guy. I was also the most mature among my peers. I would have been over the moon for some girl I liked had the willingness to make the first move. Us shy guys play out so many scenarios in our head that we paralyze ourselves.