Is he saying this just to get ?

He's super busy... which I fully understand. But he says he really cares for me. We started having and now all of a sudden he says that he's not sure if he believes in complete monogamy... He believes in open relationships. He told me this to so as to not "lead me on". But then on the other hand he's super sweet, attentive and just all around amazing. I told him the open relationship idea is NOT ok with me and he totally understood. So as of right now that's where I stand. Do I believe him when he say's he'll be faithful to me? Or is he saying this just to get ?