There is a slight possibility that this guy just like to flirt, and leave the girl high and dry, or he could just be dumb with women and you need to make it known that you are interested in more than being good work buddies. I mean some guys are great at opening and doing follow ups, but they can't close for the life of them. He may also be married or something and that goes back to the flirting thing. Try talking to maybe the office gossip about allot of people, not him specifically since she'll know somethings up, but that's how you can get to know him a little better without confronting him right to his face, if your worried about doing that. But honestly I don't know enough so maybe he is just a bad closer and he doesn't know when he can actually ask a girl out without being rejected, so you might have to just give him that extra nudge, guys hate being rejected just as much as women do. But don't worry too much about the married or the flirt thing, those I think are allot less likely than the dummy and the bad closer thing.