It will be difficult for me to say exactly what will happen, but as far as your son moving up, if he is not mature, he probably will fill left out in the older group, and yes it is possible to move up. It will depend on how many kids are on board, as to if he can move.
We started cruising after our kids got to old for the younger group, but I have seen them throughout the ship, doing all kinds of activities.
The older group is basically the teen club, and things related to it.
Tell your son to go to the meet and greet when you first board, and hopefully will find someone his age to hang with during the cruise.