That sucks. I am sorry you are going through this.
I see two common misunderstandings of polyamory relationships. One is that if there is a break-up and the person is in another relationship, then it should not hurt because you are still in love. The other is that since polyamorists can be in more relationship at a time, that we do not devote ourselves to those relationships very much. These are bother very false, but may seem reasonable from a monogamy point of view.
My guess is that he either met someone who says she wanted monogamy and he had to break up with you. Or he felt threatened by other lovers (or potential lovers) in your life. But that is a wild guess since I don't know much about the relationship itself.
If you want to bother, just let him know that he hurt you and leave it at that. He has to deal with his consciousness in hiw own way.