Wow. That is tough. I come from an academic background where quoting like that without giving a reference is a big no-no. However, it could be that he just really liked how one person put it and felt that it best described how he felt. Or it could just be a quick way to make an essay that would appease you. (Though the easiest would just be a straightforward copy of another article.)
You could talk to him in person and ask him verious things about how he feels. If he virtually says what he put in facebook, then maybe it is legitimately what he believes and he was just taking shortcuts. If not, then maybe he is just trying to convince you of something he may not have fully bought in.
It sounds a lot like you have very little trust in him. Are you trying to decide if this relationship is still worth saving? Or are you trying to decide if he is ready for you to be in a polyamorous relationship? Or are you wrooied he has been seeing someone else behind the scenes to explain his change of heart?