I think you are making a big mistake planning to leave your manual chair behind. In my experience most ships will not lift a power chair onto a tender, so you would be restricted to the ship for any tender ports. In addition, you are not going to find curb cuts, ramps, or other accessible features ashore, so even at dock ports you will be very restricted where you can go with a power chair. Lift-equipped vans are virtually unknown, so if you cannot transfer to a cab and put a folding manual chair in the back, you are limited to the area around the pier. In addition, as you know, power chairs can break down, and with no back-up you would be reduced to having no chair, or trying to use the totally crappy on-board emergency chairs only. You may not be able to take these off the ship, and may have to share with another passenger. Of course you could rent a manual chair, but this may not fit him either, and it seems a waste of money if you have a perfectly good manual chair at home that actually fits him. There is no extra charge for taking your manual chair on the plane (gate check it with the power chair). We use ours as a luggage cart to get to the gate with all our carry ons and this works out well.
Keep in mind that if either the cruise ship or the hotel offers shuttle service to the ship for ABs, they must provide accessible transportation at the same rates for those who need it. I would check with NCL and your hotel about shuttles (don't tell them you are inquiring about accessible shuttles until you find out about the regular ones). If they do not provide it, you can check with one of the local companies about availability:
Mears Transportation
Address......324 W Gore St.
City.........Orlando
State........Florida
Zipcode......32806
Country......USA
Phone........407-422-4561
Town and Country Coaches
Address......PO Box 22096
City.........Lake Buena Vista
State........Florida
Zipcode......32830
Country......USA
Phone........407-827-6350