Thanks for reading this - I need support right now,
My first thought is that he has started dating someone seriously. Or, it could be that he has felt uneasy about your FWB situation for a long time and now wants to extricate himself from it. It doesn't sound to me like he's accusing you of anything. You're the one who has been keeping it going, it sounds like his interest level was not up to yours, even if the sex seemed amazing when you did get together.
I think FWB situations can be a very tricky thing, and they just don't follow the same "rules" that a regular relationship does. Anything goes, and many times you have to define it as it goes along. That can be a good thing, and it can be a bad thing. I think very few women can successfully have a FWB, many women become attached and want more.
It's up to you if you want to contact him again, but I'm not sure what you wish to accomplish. If you really want closure, then you could send a text stating that and truly wish him well--after all, he's done nothing wrong IMO.
I think he has made it clear that he does not want any more to do with you, so no, you should not contact him again. Usually in any kind of break up, the one left is not given any real explanation, nor is one really owed. He just wanted to move on, so that's it. But my guess would be with the other poster, to cut it off completely, he probably has a girlfriend now. Good luck and sorry it didn't go the way you had hoped.
I don't know that he "suddenly" feels that the FWB isn't right.