It seems to me it's just hard for him because he's been expecting and wanting something for so long: a "normal" life progression of getting married, buying a house, having children, and so on, you know, the traditional model.
Now he found a special someone but can't marry her. So there is a contradiction for him. He wants to find someone just like you... but not married.
He won't find someone "just like you". He might find someone different, and he might not find anyone, but he won't find a you who isn't married. Is he talking of finding a special someone and still being with you at the same time? Is he longing for something more official, where you don't have to hide?
I can't know what it feels like for him, not being in his position, but it's probably hard and conflicting. Maybe he can't imagine a life of never getting married to the woman he loves, if he is mono. Maybe he can't imagine a life of not being married at all, mono or not. The fact is, you can't marry him, and maybe that makes a very big difference to him.
He might need to lay things down, between his hopes and expectations and what his life is, and decide what's more important to him. That might mean renouncing the idea of a "normal" life. Which might be very hard for him, but if he loves you, it is probably worth it, right?
I hope he will find a way to be happy with his situation, or change it.
Me: 31F, straight
Seamus: My husband, 32M, straight
Dragon: My boyfriend, 29M, pansexual, married to Fox
Fox: My boyfriend, 29M, homoflexible, married to Dragon