The sidearm delivery is actually the most natural and the easiest on the shoulder and arm, so your son shouldn’t have to worry about other things and can concentrate on his mechanics—and on throwing strikes. I was a natural, honest-to-gosh sidearmer, having discovered at the age of eleven that I had it—not to mention a pretty good little curve ball that came attached to it! Not to mention the crossfire—this is a beautiful and lethal move that works only with this delivery; I picked it up a year later and had tremendous success with it. The beauty part is that batters can’t pick up on it.
My pitching coach, a veteran major-league pitcher who was one of the Yankees’ Big Three rotation in the early 50s, firmly believed that every pitcher has a natural motion and that the thing to do was work with said pitcher to maximize his/her effectiveness and capabilities. Among other things, he helped me refine that crossfire and had no reservations about teaching me a lot of advanced stuff he felt I needed to know, because I was really interested, wanted to know and was willing to work at it. It is to be hoped that your kid’s coach has the same good sense in his head. So let him go ahead and throw sidearm—there were, and are, a number of great pitchers who use(d) this delivery: Spud Chandler, Jeff Nelson and others of that ilk. 8)