I get you, before being with my bf he was out of a relationship for about 2 weeks i thought I was a rebound too but we are still together after 2 years. We didn't know each other like you guys.

Someone people are like that thinking of other people while being in a relationship other don't. Maybe he says that because it's true or you are just convenient for him and get nice sex.

If you are disgusting just get out, I ghosted multiple people because of this reason. Nothing of value would be lost.

Also I honestly believe 100% he didn't tell shit to his FWB about you because even if they were like that their sex would be whack after talking about other people and would make them feel inferior, lol.

If you have mutual friends you can ask them, but be careful. I asked some of my friends in the past about certain guys they knew and they saved me from the worst situations ever. Hope it helps