@Mac dad I highly recommend heaters right down the middle of the plate. Leave 'em high if he's pitching against a big lefty.
Signed,
Parent of a position player also attending AC games
OK, all joking aside, IMHO the best thing for your son to do is play within his game and his strengths. If heat is his thing, he should make sure he showcases his velocity with some consistency over the innings he pitches. If placement and a good mix of pitches (or arm slots or whatever it is) are his bread and butter than that should be his focus. If he has an opportunity to show off some heat he absolutely should but not at the expense of the rest of his game.