Something to remember about
ECV's is that it makes getting around on Disney transportation tougher - with lifts - getting it secured - etc etc
For older people they like the independence and mobility that a
scooter gives them - but for a younger person - who typically doesn't like to wait - and some who don't like a fuss to be made over them getting a scooter could be a nightmare. Plus for an inexperienced 'driver' it can be VERY hard to navigate the parks avoiding people and obstacles alike.
You may want to consider just renting a manual wheelchair - not only will you save a ton of $$$ - but it make be a better option for you. As long as family members are willing to take turns pushing and your DS is willing to wheel some on his own - it's something you may want to consider.
We took my parents in 2008 and my DM is in a wheelchair full time, and she wanted to ride the train to the front of the park - so I volunteered to meet them at the Main Street station with her scooter - getting that scooter through the park - especially down main street was a nightmare. So I'm speaking from experience when I say - before spending a ton of money
renting a scooter - be sure it's the best option!