If he absolutely needs one, you may find the best thing is to rent one from off site. All of the places will deliver the
ecv to your hotel and pick it up when you are done. Here is information from some of the sites DIS people have posted about renting from:
Care Medical:
http://www.caremedicalequipment.com/
Walker Mobility:
1-888-726-6837
www.walkermobility.com
RANDY'S Mobility is in Kissimmee 407-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html
Also lists respiratory equipment
It is less expensive to rent one from outside WDW than to rent one each day from the parks. If you are staying at a WDW resort, almost all the buses are wheelchair/ecv accessible. All of the monorails are accessible. If you are staying off site, there are
ecvs that will break down into manageable size/weight pieces to fit in a car trunk.
If you rent on a daily basis from the parks, the ecvs are first come, first served, so you may not get one. If you change parks, you can show your receipt and get another one without paying again (if there are any at that park).
I'm sure you will get more replies from posters who have rented ecvs, but this will get you started. If you think of more questions, feel free to post them.