Can a crush on someone go away just like that?

I don't mean to be confrontational in any way here, but I'm having some trouble understanding the reason for your question. If you scarcely know him, rarely see him and don't seem to have feelings for him of your own (taking into account your update); then why is it relevant whether he still has a crush on you?

Or am I just missing the point and that was just a segue into pondering about the durations of crushes in general?

Sometimes, for some people, a crush can go away so quickly. That certainly does not mean there is anything worng with you. People change. And sometimes it can happen right under your nose. So don't worry, I'm sure nothing is at the root. Sometimes things just happen.
As for me, the last crush I had on a girl/woman was 4 years ago. And I still have a crush on her. I don't get to see or talk to her anymore (except FWD emails, like jokes and religious emails), but I think about her everyday.

It can be a combination of things. He could be totally in love with his girlfriend (she's opening up, they become more intimate). It could be because he didn't find you all that more interesting after his initial impressions.

But to answer your question...yes I guy can lose a crush on a girl very suddenly. It happened to me when I found out a girl I liked showed me her new boyfriend. I was like...*I don't like you no more*

Human emotions are as flaky as snow, and they can change at the drop of a dime.
Most likely he and his girlfriend developed stronger feelings, especially if they are still together. Also, since you weren't in his presence for a long time, those feelings could absolutely dissappear. This is not to say they couldn't reappear.
But I'd wait until he is girlfriend-less to get your hopes up. Otherwise a number of things could happen, and most of them ending poorly for you.

Also, consider karma...one day you will have a boyfriend!

you need a disrtraction because your not emotionally attatched to him it will be easy for you to get over him,. find a new cutie someone fun to flirt with. once you pay no more attention to your old crush you'll be more happy. I always could go back to an old crush..but when there is someone in their place.. buhbye crush