I have a terrible fear of heights and Soarin' is my absolute favorite ride at WDW! I actually love to sit in the top row! One time I did look down and wished I hadn't, but looking at the screen doesn't bother me at all. Like you mentioned op, I think of it as a way to experience things like hang gliding than I will never do in real life. Just knowing I'm buckled in (he could probably even use the additional strap they have for kids) I'm not bother at all. It does freak me out when my kids try to lean forward, but I'm just fine sitting back. Love, love, love it... could ride it all day long!
I think he would have been fine with the visual heights part if there had been a more secure feeling in the seat, like a lap bar or something. 

Just get out of people's way LOL. Regarding the former post, actually I feel like I'm leaning back some and fit great, maybe it's because I'm so relaxed. Another pp, you're not going anywhere, why would anyone feel the need of a lap bar or shoulder harness. I can understand with "real" fear of falling like on coasters, etc. to have harness lap-bar type things for safety.