A lot of what you said about your relationship reminds me of my relationship. Not saying that your fiancé will or won't do it, but I had a bunch of running gags going up to my wedding with my now wife. She has this thing where if somebody says "yawn" she HAS to yawn. You can do it like 5 times in a row and she'll have these giant yawns over and over while her eyes water. My first name is John, I kept telling her that in the vows part I was going to say "I yawn, take you ______" and make her yawn in front of everybody. She was so horrified by this possibility. Of course when the time came I didn't do that because she explicitly said not to, but it was funny to make her think I would. Another one was telling her my groomsmen were playing 9 holes of golf before the wedding (we held the ceremony at a golf course) and that they would probably be sweaty and drunk at the ceremony. Again, didn't happen but what's a little bit of fun and games? At the actual wedding I was serious when it was time to be serious, and I got in a few good punch lines at the reception when we did our speeches. She twirled me during our first dance and it looked ridiculous, but hilarious. All in all it was the best day of my life and I'm typing this while she takes her shift driving us home from our honeymoon road trip.

I sincerely hope he's just trying to make you worry a little bit leading up to it and he will buckle down and be serious when it's time to be serious and respect your wishes when the time comes. All I heard from people at our wedding was how we have such great senses of humour and that we're so perfect for each other because we spent the whole time laughing together. Hopefully you guys have a similarly great day, wish you the best.