A mile is a significant distance..... I haven't seen how big his scooter is. But have you considered a "transport wheelchair". I used this for mom in our travels and it made the trip so much more manageable.He can walk some, and in fact his physicians want him to. Its just when it gets to over a mile without rest he needs the scooter.
The cruise ship is big. Depending on how far he needs to walk for food, shows and exploration.... I should add that a wheelchair/scooter will be handy on days with the unprotected rough seas between Ketchikan and the BC inside passage.