First, welcome to the forum.
I haven't been on here long, but my experience, so far, suggests you've come to a good place to find some of what you need. Here, I think, you'll receive sound (and sometimes stern) advice here, as well as understanding and compassion.
Others will have better answers - and better questions! - for you, but here's my take on it.
What, exactly, is he asking for? "Polyamory" can mean a number of different things. Does he want to find another woman to join you in a triad? Or does he want to be able to date other women on his own? It is even possible to be a "mixed" couple, in which one partner is polyamorous while the other is monogamous.
All sorts of things are possible, but the indispensable ingredient is the knowledge and freely given consent of all involved. That seems to be the rub, as you see it.
What do you understand "polyamory" to be? What you write here suggests that to accept polyamory would be to give up on your marriage: he wants you to consider polyamory, you say, but you don't want to because you still think your marriage can be saved . . .
For the record, it is possible to have a strong and healthy marriage in which both partners are open to close relationships with others.
You state here that the only alternative to polyamory, in your particular case, is emotional detachment or divorce. Is that your understanding of the alternatives, or something your husband has stated or implied? If your husband states it, did he intend it as an ultimatum ("poly or else!"), or are you just hearing it that way?
It might help to know more about the state of your marriage before your husband brought up polyamory. Were you already heading for a break-up? Or were you generally happy together and able to communicate openly and effectively?
If your marriage was in trouble, then becoming polyamorous will not, in itself, save your marriage. In fact, it could add a kind of complexity that would call for even more openness, trust, and effective communication than a solid monogomous marriage.
Certainly, one partner trying to pressure the other into polyamory won't help things, if the marriage is already going through a rough patch.
I think I'd need to hear from your husband directly on this.
Is the point of polyamory, for him, that he can find someone shiny and new to add variety and spice to his emotional and sexual life? (That's what's suggested by "a new 'dynamic'", at least as I read it.)
Or is it that he is really convinced that human beings are capable of close relationships with more than one person at a time, and he wants to do the hard work of cultivating that capacity in himself? (That's what's suggested by the language of having "more love to give," as I read it.)
The second option is more in keeping with the ideas of polyamory than the first, which could just an ordinary mid-life-crisis kind of thing.
Okay, you're jealous. That's not necessarily a bad thing in itself, and not really all that surprising, given the assumptions and expectations of our culture.
But what's underneath that jealousy? There are lots of threads on this forum about jealousy, how to understand it, how to work through it. I think they might be useful, even if neither of you follows through on polyamory.
For me, one of the most liberating aspects of polyamory is that it takes a lot of pressure off me and my wife alike: neither of us has to be everything the other could possibly want. If being with another guy brings her a particular kind of good experience I cannot provide - and we're not necessarily even talking about sex, here, but other kinds of activities and interests - then I can be happy my wife has found more ways to be happy. She and I still have a solid relationship, and there are things we can be and do for one another no one else can . . . but I am not her be-all and end-all, nor is she mine.
(Some folks in the poly community have coined a term for the happiness a person feels at the happiness of a loved one, even if that happiness comes from someone else: compersion. I don't much care for the term, but I like the idea it embodies. You might try a tag search of compersion on the forum here to find out more.)
This is really the crux of the matter.
You say your husband "asked" you to "consider polyamory". Again, is he insisting? Is there a tacit ultimatum that, if you don't, your marriage is over?
Or is it just that talk of polyamory makes you feel afraid, insecure, jealous . . . and so you feel as if you are being pressured, even though your husband is really only raising the possibility of thinking about relationships in a new light?
Again, it might be helpful to hear from your husband on this. If he's serious about exploring the possibility of polyamory, he could learn a lot by being on this forum . . . including advice on how to approach discussing it with you in a way that won't leave you feeling pressured!
For your part, it might be worth your while to at least learn as much as you can about the idea of polyamory even if, in the end, you choose to remain monogamous. At least then you'd have more of a sense of what your husband might be asking for.
If he really is trying to pressure you into something you don't want, though, then there are deeper communication and power issues in your marriage that might call for counseling.
This is a very important postscript. You've got one very big life change coming up - and believe me, I know the difference a second child makes!! - so this seems an especially bad time to be considering the possibility of another major life change. . . especially one that (on the face of it) goes against much of what our culture teaches us about love and marriage.
If your husband is serious about polyamory, and if he's serious about your marriage, it seems to me he should be willing to put off deciding anything about opening up until after the child is born and your life together has restabilized, at least a little.
(If he's pressuring you, or is determined to be polyamorous right now, with or without your consent, then, again, that's a much deeper problem.)
In the mean time, you might benefit from exploring the ideas discussed in these forums and in the broader literature of polyamory - there are threads that provide links to other resources. Even if you ultimately reject polyamory for yourself, I think the ideas discussed here are useful in building and sustaining healthy relationships of any kind - especially ideas about honesty, communication, and consent.
EDIT: Wow! Five or six replies appeared while I was writing this! I hope it's still relevant. Again, if your husband is serious about polyamory, you should get him to join this conversation. We might be able to offer better advice if we could get his perspective, as well.