I'd add the Tomato or the Columbia seat to the consideration.
http://www.adaptivemall.com/safetycarseats1.html
Two of my special needs friends (both 10 yr olds) have the Tomato and have been very happy with it.
cons: It is a heavy thing and requires heavy duty tether after a certain weight (50-60 lbs, IIRC).
pros: It is airline approved. Can be used as a positioning/booster seat by attaching it to a regular chair. Both features are handy for travel (even if it's just to homes that aren't w/c accessible).
AND I second the suggestion to look into a customized van, depending upon the child's need for positioning and support and his ability to assist with transfers in/out of a vehicle.
While travel in a seat is safer than in a WC, once a child reaches a certain height/weight, transferring between the two is very hard on caregivers.