If a person has trouble walking, the Mayan ruins tours should NOT be considered. The ruins require walking to see them. They are not someplace a wheel chair or cart can be used and can be physically taxing to see.
The tours you book through the ship will state this. If he truly has issues with walking (if he can't walk more than a quarter mile) then I would reconsider taking this tour. If he can walk with the add of a walker or can or something and thinks he can handle it then that may be a different story.
I have not used the tour company you mention, so I can not give you any advice there.
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
NCL Dream October 2006, RCCL Mariner OTS June 2007