I think you are wise to consider a wheelchair. The distances to walk at WDW are immense and most people don't realize it until they are there. It's over 1 mile just to go around the lake at WS. I don't think your FIL will get a lot of dirty looks. The people who do are generally younger, healthy looking people. They get looked at as not needing a wheelchair since they have an invisibile need for the wheelchair. There is a false belief that people in wheelchairs/
ecvs get special treatment or avoid waiting in lines. That was true 15 years ago when we first started coming to WDW, but it was because the lines were not accessible, not to give any special benefit to wheelchair users.
Anway, if you think he will only need a wheelchair some days, you might be OK renting at the parks. They have wheelchairs and ecvs (motorized scooters) available first come, first served. Here's a
link to the DIS page about mobility disabilities which has info about renting from the parks and also a list of rides with "Mainstream Access". Most rides/attractions and all the fastpass lines are "Mainstream Access" which means that people with wheelchairs or ecvs wait in the same line with everyone else. For some shows, you will wait with everyone else in line and use the regualr entrance. Some rides have mainstream access, but the boarding area is not accessible and wheelchair boarding is at the exit. In those case, wheelchair users are directed to the wheelchair boarding area just before the regular line gets to the boarding. WDW policy is to let the wheelchair/
ecv user plus 5 members of their party use the wheelchair boarding area. It would usually be OK if you have one or two extra children, but if you have a party of 10, you will probably be asked to split up. Some of the wheelchair boarding areas are small and don't have room for that many people.
If he can walk Ok, you might want to use the wheelchair to get from place to place, but have him walk in lines. That would help prevent him from getting too stiff from sitting for a long time. When you get in line, just ask the CM how long the distance is, how long he will need to stand in line and if there are any things you need to avoid (like stairs) in the regular line. If there are, you can bring the wheelchair into line (see the link for rides where the wheelchair can stay with you). Here's another
link that should help. It's to the official Disney site and includes link to short descriptions of the rides. There's not a lot of information about boarding, but it does list those with moving walkways, which are a problem for some elderly or people with disabilities. On those rides, the wheelchair entrance allows you to board at the exit, which allows a little extra time for boarding. On most rides, the moving walkway can be slowed or even stopped if you board at the exit (Peter Pan is the only one that can't be stopped).
Many people choose to rent from an outside company because you don't have to worry about getting a wheelchair each day and you have it for use at places outside the parks.
Here's a list of off site places that DIS posters have reported renting ecvs and wheelchairs 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
This is just a start. Ask as many questions as you like; that's what we're here for.