If you are there within a few hours of opening, it should not be an issue.
If you plan to be arriving later or if you plan to park hop, there may not be any available. If he needs something and everyone is OK with a wheelchair if no
ECVs are available, you will be OK.
If you are driving to the parks, the shortest walk will actually be parking in the regular lot and using the parking lot trams. The handicapped parking lots do not have tram service; there are courtesy wheelchairs to get from handicapped parking to the
ECV/rental area, but no guarantee there will be any to use.
When thinking about renting or not, also consider your resort. The WDW resorts are large and the walk to get between your rooms and bus stops, pools, restaurants may be farther than he is thinking.