I should also say, I think it's PERFECTLY reasonable for him to say he doesn't think he wants or can handle polyamory. What I feel is UNreasonable (to me, in my mind) is for him to want polyamory for HIM but not you. He's still entitled to want whatever he wants (as are you), but I don't think it's fair of him to expect you to be okay with inequality.
And.... if he doesn't think he wants polyamory.... then why is he still actively looking for girlfriends on dating sites? I don't understand that.