Introducing:
 

I am not greek. I just have a simple question.
My friend is crossing Omega Psi Phi Fraternity Inc. in a couple of weeks, and I want to know a discreet way to ask him for his line number so that I can make him a poster and buy him a gift. What are some discreet way to ask?

Buy him a gift/make a poster that DOESN'T have his line number on it. Or give him a card saying "this is an I.O.U. for [insert piece of para here] complete with line name and number." Then go shopping for it with him.

Don't encourage your friend to reveal information that needs to stay secret for the time being. He made a promise to his future brothers, don't ask him to break it.