You can go to Guest Services the first day as soon as you board the ship and fill out the paperwork. It will list all the port stops on your cruise and you will check the ones he can get off the ship.
That will be the back up plan, but I would rather have it signed and done so I can take it with us since we will be arriving at different times. I like to get everything done at one time and then I can go enjoy the boat while I wait on the rest of our party.