Hello, Christine! When my mother comes with us to Walt Disney World she rents an Electric Conveyance Vehicle (ECV) also. We find that it helps keep our group together because otherwise she can't keep up. You can
rent an ECV for your father as long as you have a photo ID on hand. It's good you plan to arrive at the park early because ECVs are available on a first-come, first-served basis. You didn't mention which park you were planning to rent from, but if you go to the ECV page on the Walt Disney World website, you can find the ECV Rental locations in each park if you simply scroll to the bottom of the page. It has maps for each park and Downtown Disney located under 'Find ECV Rentals'.
If you plan on renting an ECV with an outside company, I suggest the Buena Vista Scooter Rentals. Their prices just can't be beat, especially if you are going to need a scooter for more than three days.
I hope this helps, and if you need anything else, please come back and see us. Have a magical Walt Disney World vacation!