Does he know how to sit ski already or are you talking about him learning to sit ski? If it's the former, call around to the snow cat operations in the state. I know some of them have experience in this area.
If it's the latter, Winterpark has an internationally recognized program in adaptive skiing. I've met several people who have come from Europe to learn to sit ski there. There are other mountains that have sit ski programs, I just know that Winterpark is one of the best there is.