What sort of assistance does he need?
Disney CMs are not allowed to physically lift or assist anyone, other than providing a steadying hand at times and moving the wheelchair out of the way after you have boarded a ride. For the rides with moving walkways, the CM can slow or stop the walkway if you board at the exit (except for Peter Pan). If he is using a wheelchair or
ECV, he will be boarding at the exit for those rides (usually after waiting in the regular line until just before boarding).
Anything more than that, the person needs to provide their own assistance.
If you go to the disABILITIES FAQs thread, there are a couple of things that will be helpful to you. Post #2 has information about renting
ECVs and wheelchairs. Post #3 has some helpful links, including links to the Guidebook for Guests with Disabilities for each park and some past threads about touring with a wheelchair.