This guy who is been texting me trying to hang out with me and have sex just told me he has a fiancé I don’t know her she doesn’t have any social media what should I do? What do I tell him? He says it’s an open relationship but I dont know If I believe him

Edit*: I know to “drop him” but what do I say?

Edit times two. Thank you all. I just blocked him so I hope he never contacts me again.