gobait,
luckily I ain't in a chair anymore but know how he feels about wanting to do it him self. If he his having a big company build it I sure they can do anything he wants if ya'll are trying to do it your self I have a few ideas on the topic
I am currently setting mine up with a finger throttle because I still have trouble controlling my leg and less pain. I a just am going with a jet ski finger throttle and cable but brake handle etc.. would work that will get all his control to his hands.
getting on and up to the seat him self (again there may be a fancier way) I say at least have a deck on the right so he can slide for the chair to the deck then a 45 degree ladder basically he can move him self one step at a time the top should get him next to or in the driver seat, this sounds weird but when I was still in a chair we build several deer stands like that for me about 25' tall so it does work then he can do every thing him self.