ex is still in love with me

I dated a guy 3 years ago. I was so in love with him, but it wasn't the right time for him. We broke up and then I got with another guy a few months later who is now my fiancé. My ex recently started talking to me again. (My fiancé knows that we talk.) He just told me that he is still in love with me. What am I supposed to say? He knows I'm not going to leave my fiancé. I just don't know what to do.