Welcome back, Diane!
Your vacation at Disney's Yacht Club Resort is almost here! Touring the theme parks with a bad knee can certainly put a damper on your vacation. I am sure your husband is grateful you are looking into ways to make his visit easy and as pain-free as possible. My mother has knee issues that impair her walking so she often uses an Electric Conveyance Vehicles (ECV) during our vacations. She loves that she can still keep up with us without worrying about being tired or in pain.
There are
ECV Rentals available in all four theme parks and Disney Springs on a first come, first-served basis. Reservations are not accepted. These rentals also must remain in the area in which rented. So, if your husband needs an ECV to get to a theme park, this may not be the best option for him.
You may want to rent an EVC from a Disney featured provider. This is what my mother does, and she loves that she has no limits as to where she can take her unit. She has used Buena Vista Scooter Rentals several times and is always pleased with their service. We like to make the reservation about a month before we arrive to arrange the details. They will then deliver the unit to our Disney Resort hotel's Bell Service the day of our arrival. On the day we check out, we take the unit back to Bell Services where the company will pick it up. You can find a list of Disney featured providers at the bottom of ECV Rental page linked above.
I hope this helps with your planning! Please visit us again should you have other questions before your May visit.
Missy