If for some reason you decide to stay together, is his variation of poly behavior that something you want to handle? Him rushing off to do as he pleases, being insensitive to gush about a new person when you are obviously having trouble with how fast things are moving, informing you he's off to stay with her instead of having a conversation about it?
I couldn't do poly with a partner who didn't remember to be kind to me during the process. I have to agree with the previous comments that it sounds like you knew what you needed to see if your relationship was going to work out or not, and he is not interested in this time at doing that work. Even if the NRE does fizzle out and he realizes he was a jackass, you probably have months of dealing with shitty behavior from him in the interim, and would have that much more trouble feeling you could trust him to be a good partner if you attempted poly again.
It does sound like you already know what is best for you and what you need.
Happiness will never come to those who fail to appreciate what they already have.