As someone who uses an ECV in the parks now, I highly recommend it. I rent offsite, so I can use it to and from the hotel. I originally planned on renting a wheelchair the first time, but you'd be amazed how many slight inclines there are that you don't feel when you're walking. You'll definitely feel them when you're pushing a wheelchair!
But
@LeiaOfAlderaan makes a good point that it can be stressful to drive an ECV in the park, especially when people are constantly looking at their phones. I've had people run right into my parked ECV, and people will have a tendency to "quickly cut through" in front of you, not realizing that it can be harder for you to stop than someone walking. This will be true with a wheelchair too though, so the person pushing will have to be extra vigilant.
Regardless of whether you choose a wheelchair or an ECV, just know that your father can park it at anytime and walk until he gets tired. I mainly use the ECV for the long treks from land to land, parkhopping, and to and from the hotel. It's parked the rest of the time while I walk around and explore.
Wheelchair rentals in the park seem fairly easy to get.
Disneyland's ECV's are bulky, harder to use, more expensive, and run out fairly early. So if you choose an ECV, offsite rentals are usually the way to go.