Disney cannot legally prevent a person from bringing their own
ECV into the parks, as long as it conforms to the ADA guidelines for safety and size (basically it cannot be too big or about to light itself on fire). They CAN limit who is allowed to drive their rentals. Same with most
ECV rental companies. It will be in the user agreements about the age of the operator.
Completely aside from liability or the driving age in Kansas- driving an ECV is exhausting. I have a sister-disorder to Marfans and I also have a spinal fusion, and I am really not comfortable driving an ECV for any length of time (I am 31). I have a manual I prefer because it requires a lot of body energy, not brain energy.
Something else to consider - it can be dangerous to drive
ECVs (regardless of age) when on some medications. I know I was on heavy opiates for months after my surgery. I have no idea what your son's needs are with meds, but it is something to note.
I think you should look into something he can try locally and bring with him. That way he can practice at home and decide if it is something that is ultimately helpful.