Thanks everyone!! I knew I could count on the DISers to help me out. He was hit by a car while riding his bike as a kid, flipped over it and landed on his tailbone which jarred his neck. He has lower back issues also but his neck always seems to be worse. He doesn't see anyone about it regularly, although he did go to a chiropractor for a number of years. He quit going when it would give short term relief and reached a certain level of improvement but never beyond that. I think DS9 is going to really want them to ride it together, so knowing to sit up front is a great help.
Like others, I also avoid Space Mtn. I klunked my head on it as a teenager and that was it for me. It was the first and last time I ever rode it.