I'm also curious about the 50lb flywheel thing. I've heard that before but never really been sure what it meant.
I started on a 20 year old yamaha xt550 dual sport when I was 17. It would probably have been an ok bike for your son, as it was way too big for me being only 5'7. Just another option if he's not too into cruisers.My son's first motorcycle will have to be a cruiser with forward controls. He is too tall for a crotch rocket.