I agree with everyone else; you are doing good and baby steps is a wise solution in your situation.
What I found very familiar in your story is the fact that you can first meet a brick wall with introducing new ideas (not necessarily connected to poly or even relatioships at all), but when people have time to process those ideas within themselves, they might completely change their minds. Or maybe not completely, but anyway open up to new ideas at some level. It took me a while to realize this. That I should not treat someone's opinions as the were "carved in stone", as that this person will always think like this. They might change as life goes on and they experience new things. (Alas, I am not the only person in the world experiencing changes in life!!)
Sounds like your husband is going through this kind of a mental process right now. If and when you one day proceed to telling about this to your friend, you should be prepared for another, equally challenging mental process that will start in his head. Which, sounds like, you are well aware of.
Just want to wish you luck with everything!