Poly woman with two ENM partners is developing a connection with a monogamous man who struggles emotionally. How to navigate this ethically?

Long post warning. I’m sorry in advance, but I really need outside perspectives. Hi everyone. I’m writing here because I want to be honest and ethical, but I’m in a very complex emotional situation and I don’t want to hurt anyone unintentionally. I 23F identify as polyamorous and value polyamory. For me, polyamory feels similar to loving different friends or loving both parents equally — love is not a limited resource, and caring about one person doesn’t reduce care for another. I currently have three partners in my life, all long-distance, and we all live in different countries. One partner 23M is cuckold and genuinely enjoys non-monogamy. He doesn’t feel jealousy. Everything is fully open: I tell him everything, show him everything, and nothing is hidden. From his perspective, this dynamic is desirable and consensual. Another partner 25M is more of a submissive in a D/s dynamic. He also enjoys the openness and my other relationships. This dynamic works for him, and he does not want exclusivity or hierarchy where he is “above” others. The partner I’m getting to know is very different. He 22M has never been in a relationship before and has very little experience with women in general. I am essentially the first woman he has ever emotionally connected to in this way. He is monogamous and has a strong fear of abandonment and replacement. He is afraid of being “just a temporary guy” or someone who is easy to replace, and he feels pain knowing he is not the only one. At the same time, I was honest from the very beginning about being polyamorous. I warned him several times before feelings developed, and he says he understands what he agreed to. He says he is happy with me and that being with me brings him more joy than pain. But he also hopes, even though he knows it’s unlikely, that one day we could be monogamous, possibly have a family and children. He is conflicted: part of him wants monogamy because it feels safer, another part doesn’t want to limit me or cause me pain. He is emotionally inexperienced and doesn’t know how he will feel in the future. We have not met in person yet, but we are planning to meet to see whether we are compatible in real life before making any decisions. I care about him deeply. I don’t see him as a toy, a replacement, or a “one-night guy.” At the same time, I cannot promise monogamy and I don’t want to lie. I am willing to stop or slow down a relationship if I clearly see that it is harming him, even if it hurts me. I don’t want to be the reason someone breaks themselves trying to adapt to polyamory. I don’t believe polyamory itself is unethical, but I am afraid of a situation where a monogamous partner stays out of hope, or because they think they will “get used to it,” or because they are afraid to lose the first emotional connection they’ve ever had. If you have gone through or are currently in relationships similar to mine, please share your experience. How would you handle a situation with a monogamous partner who struggles emotionally knowing you have other partners? I am worried about how he will process this, and I’m not sure what the future holds. I’m not looking for validation. I genuinely want to act responsibly. Thank you if you read this far.