It's actually cool that you're introducing your stepson to shaving like a man. My stepfather and I have never been on the same page, or in the same book, or in the same library for that matter. My dad had more luck when he re-married, so I call my stepmom "mom #2".
I am not surprised with your stepson's reaction. Many people are like that. When I was shopping at the flea market, one guy asked me if I were a barber. My buddies think I'm crazy.
When converting people to going straight I always use the angle that the only way you'll ever hurt yourself with a straight is if you're doing it on purpose. Otherwise you'll just receive the occasional nick, like with a regular cartrige or DE.