Bringing his own personal
ECV should be fine (provided it fits within Disney's sizing box at the bus depots) but do be aware that ECV users frequently find that they Cast Members will NOT allow them to ride their personal mobility device onto a wheelchair enabled ride vehicle, and will require a transfer.
Just want to set that expectation early, because it caught me off guard the first time I took my personal device to WDW. I purposely bought the smallest, lightest, easiest to handle device I could find, and I am still not allowed to take it on to the majority of "wheelchair rides" at WDW. There are a few where I am allowed to drive it on, but most of the time, the Cast Members require me to transfer to a manual wheelchair "for safety", and no amount of discussion will change that.
If, however, you show up in your own wheelchair, there seems to be much less pushback on using your own chair.
This is not to complain - even with required transfers to an attraction supplied manual chair, WDW is by and far the easiest place for me to vacation. (Not to mention, a favorite since childhood!) But I don't want you (the OP and your son) to be surprised if you take the ECV and then find he must still walk or use a manual chair.
It's true that an ECV will give him much more freedom; as long as he pays attention, and remembers that other Guests at WDW will *not* be - they will tend to walk into him, or right in front of him, so he will have to be more vigilant and much more ready to stop to avoid hitting someone - then he will be fine. But pretty much everyone here has at one point or another suffered from someone walking INTO them, or RIGHT in front of them because they weren't paying attention, and it doesn't occur to them that we can't just stop on a dime. Building a "bubble" (where family members walk surrounding the ECV) helps to prevent that kind of issue.
If he does tire easily, the stress of driving an ECV at WDW may be difficult for him; it can be exhausting for full-grown adults who are used to driving cars!