We are never truly good at something athletic until we can maintain control and form with max effort. Why not practice with max effort? I totally agree that taking something off to maintain control is a band-aid and promotes bad habits in the long run.
Tell your son to let it rip! But also have a few memory cues if things get a little out of whack in a game. Mine was always "stay bowed up" (don't fly open with glove arm) and "finish my leg kick" (don't get in a rush down the mound).