I can think of a few ways to do it...
So here's what I'd do:
1. Have him do some sort of breathing exercises before he gets up to the plate.
2. At the plate, have him incorporate some sort of rhythmic pre-swing motion. For example, slowly gripping and re-gripping the bat. Or, raising and lowering the bat slowly. Or small hand circles, but don't let these get too large.
3. Tell him to visualize "blowing" the ball out of the park.