I understand what it is to have a long-standing crush on someone who is a good friend, and the timing never seems to be right. This is one of the very things that made poly make sense to me- the existence of such connections in my life that I never got to pursue because I wasn't "allowed" to before.
I am also new to poly, and have yet to make a move on anyone outside my primary relationship, so I don't have any solid advice to share, but I can relate.
I think, as I re-orient myself with the concept that I am free to date others, that it's going to be hard not to just look around and choose people- I think this has to happen organically. If you feel comfortable enough, after all, you said he was a good friend and wouldn't take offense, talk to him about your feelings,( with him privately- it is up to him to talk to his gf or not, depending on his feelings) and trust him to respond with what is best for him. I know poly isn't going to be for everyone, but weren't you surprised when you found this way of being? Weren't you intrigued and excited? Maybe he will be too.
If not, hopefully your friendship is strong enough that it won't make things uncomfortable.