So... I know you say you don't want to end the relationship. My two cents, though: I've been in a relationship like that, and I'm pretty sure I will avoid going into another like the plague.
A big part of the reason why I'm poly is because I thin relationships should be based on desire and respect for that person
, NOT on a desire to be in a relationship and an attitude that "well, you'll do". And even if it's not malicious; by dating with the intention of becoming monogamous he's essentially saying he'll date you until something better comes along.
My case was a little different, as he started dating a monogamous-minded girl a few weeks after he started dating me. He knew she wanted monogamy from the get-go, but continued to date her. A few months in -and several weeks after we had 'the commitment talk'- he decided that she was too important, and that he wanted monogamy after all. This whole situation has triggered a lot of self-worth issues in me, and brought back all the old fears of rejection from when I started being poly.
Is my view biased? Clearly. But I think, in an established relationship, you need to be committed to try and keep it going, or it's already failed. You sound like you really need to sit down and have a long talk about what you're both expecting from the relationship, where you see it going, and whether or not you're both committed to making that happen.
*hugs* I hope this works out for you. You deserve happiness.