Hey there,
I know that that must have been a difficult conversation for you both, and I know how unsettling that can be. In short, I can't answer any of your questions, because everyone is different, and each situation is unique. These are questions you really need to be asking him. But I do recognise some patterns in your thinking and fears that I think I might be able to offer you a perspective on. You say:
I know I can never live in a relationship where I share the same man with someone else. I feel such sadness right now and need advice. I can't be with someone who feels equal love and intimacy with someone else.
Everyone has his or her own limits. Many people really are happier and more content and themselves within a monogamous relationship, and (I hope this is obvious) there is absolutely nothing wrong with that. Others really are happier and more content and themselves in a polyamorous relationship, and there's nothing wrong with that either. If you really and truly know that you yourself are not interested in seeing others, and/or know that you couldn't thrive in a situation where your partner sees others (even if you remain monogamous), then there really is only one course of action. You don't want to spend the rest of your life tying yourself in knots trying to be the kind of partner that is compatible with him. You don't want him to be tying himself in knots to be the kind of partner that's compatible with you either. Better to just accept that this is not a bridge that can be gapped, and work on separating in such a way that you can be the best friends and co-parents you can be for your son.
However, in what you wrote, you add that you can't be with someone who feels equal love and intimacy for another as they do with you. And to me, that sounds like it could be an assumption. Not every poly person is seeking to have highly committed, highly entwined life-partnerships with multiple people. Not every poly person desires equality in their connections in that way, because not every poly person feels like they can juggle that kind of responsibility. I am one of those poly people; I have one partner who I live with, and who I want kids with, and who I want to build a full life with. I don't want that with anyone else, because in my mind that requires being a super-partner, and I'm just not that person. I don't have the skills, and I know my own limits, and one is enough!
There is a balance to be struck (for me) between having the freedom to explore connections with others, and continuing to invest enough of myself in my relationship with my gf to get what I want out of that relationship. I love the hell out of her and would never want to do anything to jeopardise us, which means that when I start a new relationship with another, I do so with keeping that balance in mind. I don't form relationships with people who are looking for a life-partner, because I don't want another life-partner. I try to be as honest as I can with new people about how much time/energy I have available so that they are not disappointed. I listen to them when they explain how much time/energy they have available so that I'm not disappointed. I recognise that either of us could meet someone that makes us reconsider this strategy at any point, but that's a risk in any relationship, mono or poly. If it happens, I know we will discuss it, and who knows? Maybe we could handle it. It's not something that either of us are actively wanting right now, so we don't worry about it.
In short, I think you need to keep talking to him about what it is that he wants. He might not have a clear idea yet, but I think it's good if you can encourage him to actually think about the practicalities and his desires and how he sees this actually working, so that you have something concrete to assess and see if you can handle, rather than just thinking about ALL the possibilities that your brain is no doubt generating. On that note, if your brain is telling you that he is wanting to replace you, or leave you, I think you need to try to rein those thoughts in. He doesn't want to replace you, or lose you. I know this, because if he did, he would be packing his bags and leaving and not trying to broach polyamory with you at all. Trust me, it's far easier to walk away if that's actually what you want. I know you must be feeling angry and like he's had this big secret that he's been keeping from you. It's true; he should have disclosed this to you before you became so heavily involved with him. However, in his defence, our society is not set up in a way that offers you many chances to question whether you are poly or mono. We are all pushed towards monogamy and many don't ever realise that there are alternative relationship structures and that you can choose something different. As a result, many people only figure this out relatively late on in their development. Listen to what he says. He is probably feeling as vulnerable as you right now.
Finally, you ask if we think he will change his mind. Some people try to open their relationship only to figure out that it is harder to manage than they expect, and they realise it's not actually for them after all. As I say, it takes a lot of effort and time and energy managing multiple relationships, whether they are 'serious' or more 'casual' in nature. The fantasy often does not match the reality. However, I think for your own sake, you should treat his disclosure seriously and not assume it's something that he will change his mind about. Certainly, don't agree to open your relationship and let him explore if you secretly believe he will change and realise he wants only you - that path has led to disaster for many.
Keep talking to him. In short bursts, with plenty of breaks if it's too overwhelming. And let him love you. I think that's the only thing that can help, but it's bloody hard to do when you are feeling so hurt and stung.