Interesting time to revoke this thread. After I wrote this, I showed him and within a day or two, him and his girlfriend split up for good.
Recently though, he met someone new and they have fallen for each other big time. She was okay with the non monogamy thing but he found that he couldn't handle her seeing other people, so we are no longer seeing each other. It sucks, but it wasn't surprising. This thing that I wrote this thread about was always a red flag: I couldn't understand why he kept monogamy as an option in the way he did by "continuing the instability" if he was so sure about being poly. I know now that was because, perhaps only subconsciously, he wasn't entirely sure. As I said, it sucks, but I definitely wouldn't be down for this whole drama again, so it's preferable that it was a clean break. This only happened two days ago.