Many moons ago, my incredible pitching coach once told me that just about any pitch could be turned into a nice changeup, and he demonstrated some for me and showed me how to throw them. Not having any kind of fast ball, I accumulated a whole closetful of changeups in addition to my slider, my knuckle-curve, my regular curve and a couple of other things.
Now. You say your son is a sidearmer. That’s what I was, and a most exasperating one at that, because when you throw from that angle the batters can’t pick up your pitches. And sidearmers have an extra—a secret weapon, if you will, that other pitchers don’t have: the crossfire, which will work with any pitch. But first things first—he needs an offspeed pitch. May I suggest the palm ball? That was my first changeup, which I picked up when I was twelve, and a very good one it is. And it is simple. You grip the ball with all four fingers on top of the ball and the thumb underneath for support, and way back in the palm of your hand, hence the name “palm ball”. And you throw it just like a fast ball. You can vary the speed by tightening or loosening the grip, or holding the ball further forward in your hand.
As for the aforementioned crossfire:again, simplicity itself. I was a righthander, so this was what I did. I would go into the windup, or pitch from the stretch, depending on who was on base. But instead of pitching directly to the plate I would take a quick step toward third base, whip around and pitch to the plate from that angle. To the batter it looked as if it were coming at him from third base, and he would usually try to get out of the way only to have it clip a corner for a nice neat strike. Your kid, being a southpaw, would take that step toward first base, whip around and fire from that angle. If I weren’t an arthritic old poop I could demonstrate this, but this explanation should do it. Ideally the kid would get a catcher—with a mitt of course—and work on it, first from flat ground and then off the mound.
Another good changeup would be the circle or “OK” change, with the thumb and index finger on one side of the ball forming a circle and the other fingers on top of the ball. Alternatively, if he has trouble with that he could go to the “c” or inverted-circle. Again, you throw it like a fast ball—one thing to remember is that you have to throw all pitches with that same motion. No sense telegraphing what you’re going to throw!
Best of luck, and have fun. 8)