I'm sure that you have heard of many organizations for people like your son, but there is a very cool one up here that I think might be relevant to the discussion. It is called that BCMOS (BC Mobility Opportunities Society). They specialize in getting the disabled into the backcountry, and have quite the cool vehicle for it. Opportunities to get him into the backcountry will not end. I bet they'd be helpful even if there is not the equivalent down where you are.
http://www.disabilityfoundation.org/bcmos/
good luck