He needs a changeup, if he doesn't already have one.
My wise and wonderful pitching coach once told me that just about any pitch can be turned into a nice changeup, and he demonstrated several for me and showed me how to throw them. This kid has a whole closetful to choose from, and let me pull a couple from my own closet for him to consider. There's my own favorite, the "Bugs Bunny" changeup which I acquired at age 12 and a very good one it was. It's a nice palmball, easy to throw and to control, puts little or no strain on the arm and shoulder. And there's what some call the "C" change, a variation of the circle change but easier to throw. Then there's the so-called straight change, in which one just pulls the string on the fastball. There are lots more, but these three are the ones he should think about and maybe learn one of them.
The only pitcher I can think of who got by with just a blazing fastball was Joe Page, a Yankee reliever of long ago who might well have been the first great closer. Nowadays that one pitch isn't enough! :baseballpitcher: