Reaction to ride is hard to gauge
My family (mother, stepdad, wife, 10 yr old daughter and 7 yr old son) all rode M:S in Oct 2003. We all had different reactions.
My daughter, the roller coaster nut, loved it.
My son, who likes some (usually smaller) coasters, loved it.
My wife, who loves coasters, won't ride it again because she felt dizzy/slightly nauseated. Perhaps age is catching up with her.
My Mom and Stepdad won't ride it again, in fact it made my mother very nauseated. My mother enjoys coasters (she did RnR, SM, ToT).
I had mixed feelings. I liked about the first two minutes but became more nauseated the last minute. I love coasters but perhaps age is catching up to me as well.
I told the kids to look straight ahead and not to move around. I spent most of my time looking forward but turned left (I was farthest seat from center of ride) to make sure the rest of the family was OK. Did the turning cause my dizziness? Probably not. To tell you the truth, I was worried about the rest of the family, especially my claustrophobic wife, so maybe my initial tension contributed some.
My guess is that your son will be fine. If he can stomach any of the traditional "spinning"-type rides, he'll probably be fine.
Hope this helps.