I have seen on the dyno motors with a good combination of parts and a bad tune that were under performers. I've also seen less than optimal combinations that, no matter how you tune them, they don't run that well. I've also seen motors that ran well on the dyno but ran poorly in the vehicle due to wrong engine combination for the application, gears, tire size, converter or ????. It is possible that the 408 is running great and your expectations are too high due to what you have heard others report or "over report" about their 408's. Without measuring the performance of your son's 408 it might be hard to tell if it is a case one of these scenarios or a combination of them that is the reason for your less than impressive performance.