I'm glad to hear that his riding, given other evidence in your daily lives, is not meant as an insult to you or your hobby. That leaves us with ... the guy is a show-off, at least in this area (maybe something that goes back to what he enjoyed about riding as a kid) and that his riding makes you nervous. I think that gives a good opening for a conversation about how his cranium etc. mean enough to you that you can't stand to watch him ride unsafely. Maybe also that there are other, safer ways to have fun on a bike. If the guy is a good listener, than maybe he's a keeper after all. Good luck!