Anyone else find this odd?

Yesterday i went out to dinner with my fiancee and his parents and we were discussing the wedding. My future father in-law asked me what my dad was wearing to the wedding and i was like "a tux." and he started laughing and was like "that poor sucker no one's getting me in a tux not even when my own daughter gets married." I'm not sure what to say at this point because i was expecting him to wear a tux that matched the rest of the men in the wedding and now i'm afraid he is going to show up in something that isn't at all dressy. I'm pretty sure that my future in-laws have way underestimated the formality of the event. On his side of the family it isn't uncommon for people to come to weddings in jeans and t-shirts. How do i explain to them in a nice way that i would appreciate formal attire?