You can always ask your cabin steward for extenders. I would call your agent or PVP now and ask if they can be made available. Maybe with some advanced warning they can be made available??? If not try fruby's idea I'm sure the stewards can find extra pads or bring additional pillows for this purpose if no extenders are available.
As far as having issues elsewhere on the ship, the ship is not any smaller than any land based public area would be. So it should be "normal" for him.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013