In my experience, when it comes to flirting, ENFJs are very comfortable with themselves.
I'm currently in a two-year relationship with an ENFJ. The only time that his social dexterity is compromised is when he's in conflict. In these instances, he becomes very distant and moody. It's strange, really. Usually, when he has a little time to feel it through, he's back to being his loving, altruistic self.
It's impossible for me to make any plausible assumptions without knowing more about him... but is there a possibility that he's gay?
One of my best friends is straight, and I went a long time without telling him about my sexuality (I was too afraid of what he'd think; we were both raised in a religious environment). When I did tell him, he was blindsided (in other words, it's not always obvious).
There may be other factors that would make the whole ordeal conflicting for him. Maybe he just feels pressured? Maybe he had a bad relationship in the past? As others have (wisely) pointed out, some idiosyncrasies have more to do with background than with personality.