Well... it's tricky. I realise he can't marry you for real (although you could have a marriage-like relationship, and even a ceremony, as long as it's not a legal ceremony), and I'm guessing the issue is that you don't want kids at this point, or would want them to be your husband's?
Otherwise, it seems to me he could settle down with you... But I guess there is the thing with everybody else. I think the reason a lot of poly people are older is that it can take a while to start letting go of "what will other people think?"
I've found that I'm much happier being out, even if it was in bad conditions and I've lost friends over it, than I was when I was hiding it from people. But depending on your situation, being out could of course be very hard and have terrible consequences.
I think it's a bit silly though to wonder about that... he hasn't met anyone (else) he wants to settle in. Does he want to leave what he has for something he might never get anyway? Hopefully, he won't grab the first woman who says "yes".
Then there is the issue of, will he be too attached if you stay together? Will it be harder to break up? Is it better to just end it now since you know it's not forever?
Well, a lot of people's relationships don't last forever and that doesn't mean they weren't positive relationships. I broke up with my husband a few months ago, but I still have fond memories of us together, and our wedding was one of the best days of my life.
It might also be worth wondering about his reasons. Not saying he needs reasons, but getting married, having kids, having a house, etc, are kind of the model we're shown everywhere, and a lot of people want that simply because that's what they see. And then some of them do it all and wonder why they're not happier. Or even, why they're miserable, in some cases. So I'm not saying he's lying or doesn't know what he wants or anything, but sometimes when you don't think about what it is exactly you're looking for, well you end up not getting it. If he wants a family, he can also get it by having you guys. And it would be sad if he broke up with someone he loves and has a great relationship with to marry and have 2.5 kids and then never feel fulfilled.
But if we assume he can't find what he wants in this relationship, and that he will definitely meet someone who can bring him what he wants, I would still suggest to enjoy what you have now. In the past, I have spent time worrying about the future or wishing for things, and I forgot to actually enjoy the good things I had. I know it's cliché, but sometimes it's really true, you don't realise you're happy if you don't stop to look at it.
In the end, though, I think what you really need to do is make a decision and stop at that. You could argue about it forever and it could damage your relationship. Pick something - breaking up or staying together until he meets someone he thinks is worth is - and then do it right away and don't look back.
If you choose to break up and are both miserable, then get back together and pick the other option, and don't question it anymore: you'll know breaking up wouldn't work.
If you pick to stay together, don't talk about it anymore, don't have the same debate over and over again. You've already decided. If your relationship breaks apart, then you'll know it didn't work and you can break up then, and not look back.
I think right now, you might be too worried about which decision is "the right one" to realise there might be no right decision, but that the wrong thing to do would be not making one at all.