I'm not exactly sure why people started using CJ, but everyone else has covered that. I think now, pretty much everyone has gotten used to that name. I've been "conditioned" so much, that if the names CJ and/or Jon isn't used for the eldest son(s) names, it just seems wrong. I know Jon has been used by comic canon (Son of Superman), but I'm not sure about any other names.

"You need me. You wouldn't be much of a hero without a villain. And you do love being the hero, don't you. The cheering children, the swooning women, you love it so much, it's made you my most reliable accomplice." -- Lex Luthor to Superman, Question Authority, Justice League Unlimited