A lot of people in Disney walk around with their head in the clouds, so driving a wheelchair, even at the quieter times, can prove rather difficult. If he sometimes loses control of his chair, I think it would be a lot safer for everyone if he were pushed in his manual chair. I'm sure the last thing any of them want is for him to have a spasm, and run into a child. This would be especially difficult in lines, where everyone is packed in fairly close.
I know it will limit his independence, but it will save him a lot of worrying, too. Also, I don't know what his energy levels are like, but if he gets worn out steering for a long while, he could end up exhausted by the end of the day. He could always take the power chair for use in less crowded places, like at the resort. Or, if he has attendent controls on his electric chair, he could have someone steer for him most of the time, and drive himself when he feels it will be safe.
Can he transfer at all, with assistance from his family (the CMs are not allowed to assist with transfers, other than to hold the wheelchair still and move it away afterwards)? Quite a few rides and all of the shows can be experienced from a wheelchair, but many do require a transfer. If he can manage a few transfers a day, it might be wise to plan out which rides he really wants to do, and try to spread them out through the trip.
Some of the transfers are easier than others. To get details on what is required for each ride, here are a few helpful links:
The Mouse on Wheels has detailed descriptions of the queuing and boarding procedure, with information on the difficulty of transfers where they're required
All Ears has a photo gallery of ride cars and show seating, so that they can decide whether they'd be able to manage a transfer or not, before they get to the parks
Hope this helps!
