People have said so many good things here and asked so many good questions it's impossible to react to them all!
You do actually sound like you're trying to address this really well, and you've identified a bunch of important issues, both in your relationship with him (e.g., communication) and for yourself. But it seems to me - and I think you may have acknowledged this too - a lot of these issues aren't specific to polyamory. You're going to need to address them whether or not he wants to be poly. So I wonder if it would be at all helpful to try to shift your focus. Let the poly question be secondary for the moment, and deal as best with the core issues.
For me also the constant question of "can we make this work" is really debilitating. I understand the impetus to ask it, but it's SO wearing. I've found that I do much better now when I approach a relationship with the idea that I've decided to try to make it work, and do the best I can at that. I focus on the process - i.e., I am doing the best I can in order to keep improving this relationship - rather than on a specific outcome - i.e. is this good enough. I admit, I worry sometimes about whether I'm just wimping out on making hard decisions, but overall it gives me a more grounded place to work from.
You asked early on about benefits, about why do this. Now my position is very different - I'm married with kids, and my boyfriend is also, so we are each others secondaries. From that there is balance, also an accepted prioritization. I've been married for a long time, and that relationship has not always been smooth, and it frankly still has some big issues. But those issues aren't due to the boyfriend. And for me, being involved with my boyfriend has forced me to grapple a lot with my own issues, which makes me a stronger person. And it has given me more experience in relating, helped me learn relationship "tools" that my husband could not teach, since he also lacks them. So these can be taken back to the primary relationship. Plus I am simply happier with my boyfriend in my life, and that happiness can help support me through times that are tough at home, and give me more strength to deal with them. So it is not necessary to assume that other partners for your boyfriend will be detrimental to your relationship. They really can be a source of support as well. Plus they are a good spur against letting relationship complacency to set it; they are a reminder that we have to keep paying attention to the primary partner (which is actually very easy to lose sight of when you are monogomous).
I don't mean to make it all sound easy; it hasn't been. But it has been worth it.