I think a CB400 is a great little motorcycle to learn on.
I also have a friend (girl) who enjoys her Kawa older Ninja 500cc (EX500 or GPZ500). It's light and agile without being twitchy. It's her first bike and she loves it. It looks sporty.
Being young, he probably doesn't want to be on a cruiser, but if he does and look cool riding with his dad, a Virago 400 is a nice starter too.
Lastly, an Enduro style 400cc is great for threading into traffic and a fun go-everywhere kind of bike.
