I see several people have already suggested you check out the disABILITIES Board FAQs thread. There is in link in my signature to the disABILITIES Board, and then look near the top for the disABILITIES FAQs thread.
I would suggest renting from one of the off-site places so you have the wheelchair for getting around your resort and for getting to the parks. Even if you plan to borrow one from your resort, take the numbers along so you can call and get a wheelchair in case the resort either doesn't have one or doesn't have one that works for you (they are first come, first serve with no guarantee that they have one).
You may want to check on a pediatric or small adult wheelchair (if your child is small) and check on an elevating footrest so you can raise his foot. Also, if he does have crutches, you can rent a wheelchair with crutch holder from one of the off-site places.
I wanted to clear up a few things, though.
1)
The WDW parks and most of the off-site rental places require that someone be 18 or older to rent or drive/ride an ECV (power scooter). Some of the offsite places may make an exception for people younger than that who use
ECVs at home because of permanent disabilities, but don't want to travel with them.
Power wheelchairs can be rented from the off-site companies, but they will generally only rent them to people who are experienced users and just don't want to travel with their own.
These things are for liability reasons - it's hard enough for experienced adult who drive cars to manouver an ECV around WDW, and would be much harder and more dangerous to have a child with no driving experience doing that.
2)
Most of the lines at WDW have what is called Mainstream Access, where people using wheelchairs or ECVs wait in the same lines with everyone else. The only people who get "front of the line access" are children on "Wish" trips and similar situations.
AK and MGM were built with Mainstream Access (all the regular lines are accessible).
MK and Epcot are older parks and all lines are not Mainstream, but all the newer or renovated attractions had Mainstream Access added if it was possible. There are a few attractions that were not possible to update to be accessible, but many are.
If you used a wheelchair and were allowed to move up/skip lines, you were the recipient of some pixie dust because that's not the way things usually work. Sometimes it happens because a CM is just giving out a little extra pixie dust. Sometimes it's because of things behind scenes that are not visible to you - like the attraction is well staffed right now, but someone is going on break, so by the time you get to the front if you do wait in line, there would be less staff to provide assistance. Also, sometimes, there is an extra wait involved in using a wheelchair; if the regular (standby) line is short, the CM may "pull" you out earlier than the normal "pull out" point for guests traveling in wheelchairs. But, your wait may actually be the same as a person walking in line because you may have to wait at the wheelchair boarding area to board.
3)
If you want to avoid waiting in line, getting to the parks early and using Fastpass are good ways to do so. Having a wheelchair doesn't mean you will have a shorter wait - just that you will have a place to sit while you wait.
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