Millions of people make fun of Charles for his golf, but he continues to play because he enjoys it, right? So, it should be able to work both ways. Even if a fellow is concerned about dancing, you don't have to be great to enjoy it.
Don't forget Warren Sapp. He was a down lineman who showed great footwork, lightness of the feet and how a big, athletic man can ballroom dance. I also like to remind people that Oliver Hardy, Jackie Gleason and Curly Howard of the Three Stooges were all excellent dancers -- and they were big men (in their time, not really by today's standards).
That may not be the issue here, and you have a great attitude about it. Hopefully, he will dance with you just for fun in the right setting and enjoy that. Years from now, it may become a sport he is more interested in.
I, for one, would not be giving Evander Holyfield or Floyd Mayweather a hard time about being DWTS contestants. Neither of those fellows were tremendous, but they weren't bad. I also think they would tell you that the training/preparation part was similar to their own sport.