I do not believe Shawn grabbed Ben. How can a 15 yr old have the strength to grab a 13 yr old, throw him into the truck, throw him onto the floor and duct tape him. I am sure Ben put up a fight. Does not make sense. Shawn may have been driving but he is not the one that did the act of kidnapping.
However, I would never, never underestimate the potential superhuman strength that a person who had been held captive for 4-1/2 years could or would have when under extreme duress and in immediate fear of being killed.