I don't have time to post much tonight, but will sometime this weekend. We just got back from WDW (about our 25th trip in 19 years).
If you go to the disABILITIES FAQs thread stick near the top of this forum, you will find a link to the Disney official website, where you can find a link to the WDW Guidebook for Guests with Disabilities. That has an updated list of the Companion restrooms.
The newest parks (AK and the Studio) have the most and the best Companion restrooms. You'll also have the least people using them there because the ladies (and I'm assuming men's) rooms in those parks have handicapped stalls that a wheelchair actually fits in.
MK and Epcot are older parks and the handicapped stalls are generally too small to get a wheelchair into and shut the door. They do have companion restrooms, but people use them as family restrooms. Especially at MK, we often had to wait for a parent and same sex child to come out of them.
There are companion restrooms in First Aid in each park, but some are rather small.
If your DH needs a particular setup for transfer, some of the companion restrooms will fit his needs, some won't. So you may have to try some out and then note which ones will work. In some, there is space right next to the toilet to park a wheelchair. In others, the wheelchair has to be parked in front because there is not room to the side. (Next year, I'm going to have to take pictures, I think and make an inventory of the Companion restrooms).