I don't know of any place where they would store a wheelchair in the park. The only possibility would be something like using a bicycle lock to something, but that may not be allowed.
You may be better off renting an
ECV from one of the off-site companies. Often it is not possible to get an ECV at the parks, especially if either it is crowded or if you change parks at midday. Some information for off-site rentals are:
Walker Mobility:
1-888-726-6837
www.walkermobility.com
Care Medical:
http://www.caremedicalequipment.com/
RANDY'S Mobility is in Kissimmee 407-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html
I have used Walker a few times. They will ask for a credit card and charge it the day before arrival. They will deliver the ECV to your hotel bell services before your arrival and pick it up there after your departure. Their rates, especially for longer than a week, are much better than Disney's. Also, their scooters are much better. Randy's is less expensive, but you must be present at both end of the transaction.
You don't say if you are renting a car or what. If you are using a car for getting around, the
ECVs break down into five pieces, the heaviest about 39 pounds. They take about a minute or so to take apart and about two minutes (once you are used to it) to put it together. If you do not have a car, all of Disney Transport can handle it.
Since he has a wc, I would figure that he also has a state-issued HP hang tag; will be needed if you want to use medical parking. It will also get free valet parking at the deluxe resorts.