It sounds like he's a perfect candidate for a
scooter. They are treated as handicap vehicles, too at Disney. For someone who may be reluntant to use either a wheelchair or a scooter my best suggestion is to go to a local zoo or similar where there is enough walking to do in a day. See how it goes with it. I would say 9 out 10 fail the zoo test and need assistance. Also, 1 day doesn't compare to day in, day out at Disney World. By the way, you can rent scooters at most zoos, too. It would be great practice for the Disney trip. You can always get a scooter at the zoo even after a couple hours of walking if your FIL needs one. Furthermore, if the wheelchair is the way you want to go you can practice with the wheechair with him at the zoo. Then you will know exactly how you will feel and how it helps or not with your day there. A wheelchair definitely slows you down in my opinion. I would have a hard time keeping up with a scooter all day.
Anyway, you can practice a scooter free at most, larger, grocery stores and some, larger, retail chains. Some people may be scared to use one at first; so, this is a great way to get introduced to one. I'm sure a test pilot can handle one. If he makes friends with the scooter your problem is solved. You can rent from Disney in the parks (around $50/day) or get an outside rental one delivered to your resort for as little as $150/week which divided by 5 days is $30/day. There are a few outside places who will give you a daily charge, but it will probably be $30/day or more.
With scooters at Disney you will have to still get on the rides yourself. but you can take them through the ride queues and park them near the ride. Some leave the scooter near the ride (like by the strollers) and walk the queues. This may make him feel less conscious of using one.
It is a little tricky to ride a scooter through the parks. Almost no one will give you the right of way with a scooter. You have to sometimes suddenly stop or you will hurt someone. Children get caught in the excitement and will dart out in front of a scooter. Most people do seem kinder with the wheelchair.
There are rental travel scooters that can go in and out of a car. This would really help with Universal/IOA and Disney depending if you want to use your car to get back and forth from the parks. Using Disney's bus transportation a scooter is treated the same way as a wheelchair. You stand in the handicap line to board the buses.
There are lightweight wheelchairs you can buy that are called transport wheelchairs. They can range from $100 to $200. We have one that weighs under 15 pounds. I'm overweight and over 55 and I push one for my mom for 14 days sometimes at Disney World. Now, my mom is petite; so, you can't compare it to your FIL. You may be able to ret a lightweight model. There are wheelchairs that can go 40 pounds. I find those heavier ones to be hard for me to push. My hubby uses a travel scooter, but he helps with the wheelchair to and from the parks, but not in the parks. I ride the scooter until we get to the park.
You do not need a GAC for a wheelchair or a scooter; unless, there other circumstances that come into play like you can't be in the sun too long, etc. It just depends on your other needs as to whether one would help or not.
Again, whether you use a wheelchair or a scooter you will have designated areas for veiwing shows, parades, fireworks, etc. in the parks.
Do not expect to go commando style with either choice of vehicle. And by no means show your frustrations (if any) to your FIL when using a wheelchair for him. This is why it would be good for all of you to practice with the wheelchair at a zoo if that's your only option. At first, for me it was a little hard, but after a few days it seemed okay. Getting a wheelchair on and off the buses (or cars), etc. needs some time to get use to doing it.