Follow the link in my signature or look near the top of this board for the disABILITIES FAQs thread.
Post 2 of that thread has information about renting wheelchairs and
ECVs.
All of the companies listed there would deliver to Saratoga. The Disney Featured Vendors can deliver to and pick up from Bell Services. For the others, you will need to meet someone from the company for delivery andpick up.
I would still bring long the knee walker - it will be transported by the airline free of charge and will be useful for getting around in/near your room.
With that extensive of injury, he is probably still going to be on pain medication. It's important to handle driving an
ECV like driving a car and remember that pain medications might affect ability to drive safely and react quickly. In that case, he might be safer with a wheelchair for someone else to push him.
Talk to his doctor about DVT (blood clot) prevention. Flying is a risk factor anyway because you are sitting still with little ability to move around and get the blood moving. A broken leg and/or arm are Aldo risk factors because they affect mobility.