So, in the last couple weeks I've been doing some soul searching. I was also lucky enough to find a poly-friendly therapist in my area and began seeing her. She's made some suggestions that the boyfriend has been receptive to, and so far it seems to be really making an impact on him. I think he started to realize that the boundaries I had weren't just due to my own insecurities, but also because of his behavior. I hope we'll keep seeing progress, but only time will tell.
Thanks for the advice!