I agree bad fit can make riding nigh unbearable. At one point in, I believe HS, I was borrowing my dad's bike because mine wasn't working. My dad is about 5'10" I'm about 6'2". I adjusted his seat as high as I could but the smaller frame just didn't let it go high enough. After riding several blocks my legs were cramping in a major way and it was all I could do to keep riding.

I say if you can't convince him now, let him ride once or twice like that and when he starts complaining about knee pain or whatever ends up hurting, refer him to a site talking about proper fit and the complications caused by poor fit, for instance: http://sportsmedicine.about.com/cs/sport/a/bikefit.htm mentions how poor fit can cause pain on the legs and joints.