That is one my daughter has never been on with us. She also has CP with mixed tone, but has pretty good upper body control,
It kind of depends on your size. Usually, 2 people sit per row, but I have seen 3 if the child (and adults) are pretty small.
The back of the seat is not very high, so not much back support. If he can sit between you, you would be able to brace him, but with only 2 people in the row, I sometimes get jostled around quite a bit. The restraint is a single bar that goes across the entire seat. So, if one person is significantly smaller than the other, the restraints may not hold them very securely.
This video will hopefully give you some insight. Guests with mobility devices don’t use the queue shown here because there is no way to get the wheelchair from the load to the unload side.
With a wheelchair, the CM will give you a time to return based on the current Standby wait time. When the time comes, you will enter the ride thru the exit.