You do not need any other identification for your son. 16 and over requires a photo ID along with a birth certificate.
Yes, the twins can be pushed together to make a king and often this will already be done by the time you get to your stateroom. If not, just ask your steward.
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04