Yep. Tell him you have been thinking about polyamory, and ask him what he thinks of it. Simple as that, and let the conversation evolve from there.
Could be that the conversation will be divided into bits here and there, as he might need time to think about this or that before responding. But you could always answer any questions he may have.
I also suggest you have a look around our site at the various threads, and see what looks particularly interesting to you. Lots to learn here. Read, read, and post whenever you have any thoughts, concerns, or questions.
Perhaps even your husband would be willing to check out this site? Something to think about.
Love means never having to say, "Put down that meat cleaver!"