You MUST call to confirm that you will be placed in an accessible room. As the previous post says, all of the HA rooms are in the Lodge.
All of the buildings except the lodge are 4-6' off the ground.
All of the Disney resorts now have lifts at the pool.
If you need a room with a roll in shower, make sure to request that.
HA does not mean roll in shower across the board.
If he has a power chair, he could really have a blast around the island with all of the bike paths! Just make sure his battery is charging at full capacity - or rent one if you don't want to expose yours to salt and sand!
The shopping area across from the resort isn't so accessible. The stores are very small, and for the most part very packed with racks and displays.
Parking can be a challenge around the lodge, and parking under the lodge can be a little tight for larger vehicles.
The paths around the resort are relatively smooth, as are the bike paths around the island.