Well... one small potential problem.... he might not be single? Hahaha.
But... let's say he is....
I think you have two options:
Option one is to start striking up general conversation. Next time you see him, ask him how his day or weekend was. See if it goes anywhere from there.
Or, go for the direct approach. Put your number on a piece of paper with a witty note or something.
To be honest, I'd probably give my number, because I wouldn't care if I got rejected, hahaha. But the slower approach would definitely be appropriate too!
As for talking about poly... I don't think there's anything wrong with the idea of giving him your number, or getting to know him, then bringing up poly if he expresses an interest. I don't think there's any need to include that information on the note. You could always drop it into conversation, i.e. if he asks about your weekend, you could say, "Oh yeah! It was good - my boyfriend went out with his other girlfriend, so I had the place to myself... etc etc". I find that the more normalised I make something sound, the more normal it becomes.
Other than that... enjoy the crush! Crushes are great! Whether they lead anywhere or not