Welcome back, Susan!
ECVs are available on a first come, first served basis. So there could be some availability, but sometimes there's no way to be sure that your husband will have the accommodations that he needs. Typically there are plenty of ECVs for guest to rent, but sometimes it's difficult to determine if the demand will be higher on certain days.
To ensure that your husband is comfortable and has a Spooktacular time at
Mickey's Not So Scary Halloween Party, ECVs can be rented at a local, Orlando vendor. The ECV can be delivered and picked up at your hotel or resort. This will be a definite way to ease the worry and know that your husband will experience all of the candy-coated magic and pixie dust at the Magic Kingdom. This will also be a way to have the ECV available for your entire Disney trip.
Another suggestion would be to arrive to the Magic Kingdom a little before 4pm, maybe 3-3:30pm, just in case you have to wait for another guest to return an ECV that won't be attending the Party.
Boo to you, Susan! If you have more questions, please visit with us again!
Korliss