Carc is ranked the same way all the games are ranked. It's not game-specific and not based on players' actual scores, but rather placement.
https://boardgamearena.com/faq?anchor=faq_account_elo If you're looking for answers to some specific questions, you would have to ask. "How players are ranked" is fairly ambiguous.
The rankings you find on the game page I believe are defaulted to those from the Arena mode. (
https://boardgamearena.com/faq?anchor=faq_arena) I don't do Arena mode but have to assume it's still based on ELO for just that season's games. You can change that in the upper right corner to get all-time rankings.
On your own profile page, those rankings only show if a game has been played in the last 60 days, and I believe only if you're in the top 100.
As to playing non-ranking games, when you set up the game table, you can change normal mode over to training mode. Whoever makes the invitation makes this decision, so your son would either have to find a table that's training mode or make his own invitation. Under the option of who to play with, if it's set to manual you have to look through the game invitations. If it's set to automatic you'll want to pick the gear on the game for preferences and go into advance. It's the "mode" option.
