@eva - you are correct:
ECV rental vendors will not rent to him until he is 18 years old.
One thing you can consider (if a wheelchair or stroller won't work) is a
Rollator. Typically, it's as cheap to buy one, as it is to rent one. Most models will fold flat, and like any other mobility device, a Rollator will fly for free on all US airlines as medical equipment that can be gate-checked.
A Rollator is walker with a built-in seat, so that when you are standing in line, he can take that time to sit. It gives him something to hang on to when he walks (which can possibly help smooth out his gait), and most of them have a basket under the seat that can hold supplies, if needed.
Rollators typically can be height-adjusted to fairly short sizes if needed.
Remember also that there are First Aid stations in every one of the 4 Theme Parks if he needs to rest for a little bit. Ice water is free at all QS (Quick Service) food locations in case he has issues with dehydration.
I mention the following 2 options only because it sounds like when planning ahead for his future, one of these might be something for him to consider:
There are vendors that will sell
ECVs to young people:
Travelscoot has a "grow up" plan that utilizes their "Shopper" unit and allows for the purchase of a smaller
scooter, and the parts are swapped out to full-size as the child grows into their adult height. It is expensive, but I used one for many years until my condition deteriorated to the point where I had to move to a Fold & Go wheelchair.
There *are* also some powered wheelchairs that have an "attendant control" that would allow him to sit in the chair, and you (or another adult) to move him via the joystick control that is mounted on the back or handlebar of the unit, however those tend to be an extremely expensive option to purchase as well.
Fold & Go (which is made in the US, and is a local favorite of *many* of us here) has an "attendant control" option that could easily be removed when he is at home, or in places where it isn't so crowded that he can't control the chair himself. Fair warning, however - controlling a joystick based chair can have a learning curve to it, so be sure that you can allow some real-life practice time with the attendant control before showing up at WDW with one.
If he does have mobility equipment already, it will fly for free on any US airline, and can be gate-checked when you board the plane; that way he can just take his own equipment with him, which would be most comfortable for him.