I agree with all of the above. I am a submissive who's been wearing a vanilla suit for the past few years. My SO is not as dominant as I would have previously went for but there he was and I was willing to tone it down much to my surprise. But in the past year he's been opening up and stepping up closer to the Dom plate and I've been realizing that I'm not as committed to D/s as I used to be.
So I can see where maybe he has found someone that he would like to try on a "vanilla suit" for and want to back off from your arrangement. As far as the "I'm not poly anymore" announcement goes, IMHO, maybe he's just not cut out for it. Maybe he is a little more mono than even he realized. Again, something i can relate to: My SO just admitted that he doesn't want to be dating around anymore, that maybe he's just made to be mono. That's how he feels right now. He waivers. Which is understandable.
I wouldn't cross him off the list. Give him the time he thinks he requires but be available to talk and be friends. Offer hang-out time just to get together and talk like friends would.
It's just me, my ramblings and the elevator music in my head.
Me: bi, reformed wild-child turned mom and house maid LOL
Crux: straight/hypersexual, possibly mono?
BC: our son, 5 years
CM: second son 9 months.
Mouse: girlfriend! Status new, feelings not so much.