He'll be fine (as long as you and he understand and heed the numerous warnings prior to boarding the ride). The instructions passengers are supposed to follow are supposed to 'control' the ride to Mars. In fact, when the attraction first opened, if someone in the capsule didn't push the right buttons at the right time, the mission would fail (but just on the display screens). But so many guests weren't aware that it was interactive that Disney eventually changed this. If your son does want to 'actively participate', just tell him when the button in front of him flashes to push it.