Pitching is like golf. There are a thousand little minute movements that effect the final outcome. Going sidearm complicates the Hell out of the idea of using standardized, repeatable mechanics. It's more about feel. Sidearmers are to pitching as Jim Furyk's swing is to golf... It's not very often you can be consistent with that unless your name is Eckersly or Furyk.
Don't tell him what you are doing and go set up a net at 1st base. Go hit him 20 ground balls near the bag at 2nd and tell him to throw to 1st base as accurately as he can. Give a treat/prize for each one he lands in the target. Watch his arm angle. He should pick up the ball and throw it in the most natural slot after a few tosses. You don't want a 10 year old "choosing" to be a sidearm pitcher because he saw it on TV... When I was around 10 I decided my bat should sit on my shoulder when I was hitting because that's what Cal Ripken, JR did... It was not my natural stance and I had the worst year of my little league career, but I thought it was cool.
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If he throws that ball sidearm naturally and more accurately, so be it. Tell him to spend his time on the infield because a quick accurate sidearm release is an asset for a middle infielder. As a pitcher, it's going to be really hard to perform well as a sidearmer as deception is not a big deal for kids, location is the key. From my understanding, most sidearm pitchers were overhanded pitchers that went to it out of near desperation in HS/College/MILB.
From an injury standpoint, overhand is hard on your shoulder and sidearm his hard on your elbow. Being a serial farter around myself, just like Missouridawg, I threw some competitive pitches in high school and legion ball side arm... and it was way more stress on the elbow than a curve or slider. From an injury standpoint, I imagine a kid under 14 would be better off throwing a bunch of hard curveballs or sliders than pitching sidearm.