You should wait... If when they break up, you still want him you will realize your feelings were real...If you tell him now he may think it is just one the "I want you because I can not have you" things...And what if he breaks it off with this amazing girl and you realize it was one of those things...That coudl hurt your relationship more than anything...Or what if you tell him and he says he wants to stay with this girl...Your relationship coudl be awkward and eventually ruined...I think it is best for everyone if you just wait it out...

I'm constantly amazed by people who put their friends interests before their own. Your first obligation is to yourself and you happiness. If you think that being with him would make you happy and there is some inkling that he might feel the same then you owe it to yourself to pursue that avenue...
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Having said that tho, I'd also tell you that timing is everything so pick your moment, like after it ends with his current gf , but then definitely tell him how you feel. What's the point of going thru life wondering what might have been. One way or the other you'll have taken positive steps to find out...

To tell him now when he just got involved with someone else I don't think is appropriate. You could end up breaking them up only to find you still don't really have those romantic feelings after all. It might be you are attracted BECAUSE he is interested in someone else now and not you. You should examine your feelings to see if there is some underlying motivation in wanting him now. But stay good friends and see what happens after time. If he breaks up with this girl and is available again, you will see if the feelings are still there...

Bummer. That's just like when Rachel finally realized she liked Ross -- and he got off the plane with Julie...
I think I'd wait. If they break up, and you still want him, say something then. Maybe you just decided you liked him because you're kind of bored, and he liked you (which is always appealing) and there's no one else who catches your interest. If you didn't like in that way before -- what changed? Maybe you should trust your first instinct about dating him, which was no...

You blew your chance...Now you have to wait until he's available (assuming you actually like him)...