This is all really scary. I mean, worst case scenario, what if he is just using "poly" as a way of trying out a relationship with this woman and if it doesn't work out he still has you? And if it does work out and she bears this child or children he is so desperate for then what if he just ditches out on you and goes to be with her? I checked and you've been married for 5 years so I guess you know him pretty well by now.
Also, maybe "worst case scenario" isn't so bad, actually. If he would leave you in the above situation, probably not the loving partner you want to remain with anyway, right?
Sorry to play devil's advocate but I was just thinking it and maybe you've thought it too?
A word about him being so mean to you. It's not acceptable. I know what it's like to be frustrated, and I understand that we get angry and the poison comes up out of us when we have that internal battle we don't let out. I had this happen to me last spring and it ate me up. But we should always be careful with our loved ones, to honor and cherish them regardless of whether we vowed to do it or not. The promises in our hearts should be stronger and more prevalent than anything that passes our lips.
A word about "communication"- that is a good answer on things to need. Maybe he needs a definition of communication or why it is important. I don't know you or your husband at all, but maybe he is asking for some rules? I know when I first came into poly I thought about the word "cheating". Cheating is not playing by the rules. So if you take a card from the top of the deck on your turn while playing rummy, it's all good. But if you do the same thing during Texas Hold 'Em, you've just cheated.
Poly (my understanding anyway) has a great deal to do with tossing out some of the mainstream monogamous rules we've been taught. Rules we play by without even having ever read or written them down or voiced them. I had to laugh the other day playing online poker when one player sent another a rose and the recipient said "Thank you, it's beautiful." and seconds later, "My husband likes it, too." Subtext = "Taken. Bark up next tree."
So if he's asking for rules... why is he asking? Is it so he can follow them to make you happy? Is it so that he can find a way to work his way around them?
Often he had been often bound with fetters and chains, and the chains had been rent asunder by him, and the fetters broken in pieces: neither could any man tame him...and he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many.