There are also places where you can plug it in.
If he needs a rest during the day, you could stop at First Aid. He could plug it in there while he lays on a cot to rest or stretch. There is a First Aid station in each park. Some people also plug in while they are eating - just ask a CM where you can find an outlet.
Some power wheelchairs have an odometer, so you can keep track of the distance he goes. If his has an odometer, charge the battery fully and then keep track of how far it goes before it needs a recharge. Also keep track of what kind of terrain he was using it in - was it flat or hills, smooth or bumpy. The average WDW tourist goes between 3 and 9 miles a day. If his battery can do 9, you probably don't need to worry, but should bring the charger along anyway. You can store the charger either in First Aid or in a locker.