1) No. I'd say the exception is if he really wants to do a character build that requires multi-classing, but I'm not sure what that would be.
2) If by, "explain the game," you mean give him a general overview, then yes. It will take a while before he masters it.
3) I assume that he and his cousin are both playing in the same campaign. This is hard. I think it's good for PCs to be the same or at least similar levels, but 9th can be a lot for a new player. What do you think about a one-shot with lower level characters to help get him started. Maybe it could be a "flashback" to the start of his character's adventuring career and then do a time jump to bring him up to 9th level.