It is wonderful that you are bringing your Dad. I am bringing my own Dad, who will be 85, and, although it makes some things a little more difficult, it means a lot to me. It's important for my sisters and I to have him be part of our vacations as long as he is able to. And it just feels nice to read other people be so generous and considerate towards elderly parents.
To your question, Brevard Medical Equipment charges $100 for a collapsible wheelchair rental for 7 days, less for fewer days. There may also be a fee for delivery to the port, I am currently looking into that. That's what I'll do to solve the problem, also just walking the length of the ship and back repeatedly would be a challenge for him.