I have two pieces of advice, fwiw. I was the one who opened the conversation about poly in our relationship, although there was no one else waiting "in the wings", so to speak, at the time.
My first piece of advice is to bring up poly in the abstract first. Your husband may be more willing or able to talk about it as a concept first. It seems to me that it's much harder to be receptive to the idea of poly when it comes along with the "and there's this other person I'm interested in" conversation. Saying "hey, did you hear about this celebrity who's in an open relationship", or "did you see that Newsweek article on polyamory the other day?" may be a better way to start.
Second, and I give you this advice in the full knowledge that I do this also and it hasn't worked out that well for me--try to not have the conversation with your husband in your head first. It makes it harder to actually hear what he has to say, and, at least for me, it ramps up the emotional volume enough that I am often so defensive and on edge from what I imagined his responses to be that I can't listen as well as I'd like to the actual conversation we are having. I'm working on it, myself, and it just struck me from your post that it's something you may want to pay attention to.