My son had 9 strike outs in two innings 2 years ago as a 12 yo playing on a (poor) 13 yo team…a five and a four SO inning with 2 different catchers…and though disgusted, he didn’t point the finger at any of his teammates, just kept on throwing.
His attitude is a great thing to hear. It’s good that he understands that people are not perfect, and that he can’t allow what’s out of his control to affect his game. I think we call that “poise”.
Good news: your son can learn to throw a change-up that has plenty of action and can be just as deadly as a slider, only better, because it can be thrown for a strike.
Challenge him with that, and see how he responds.
Start him off with an easy, pitchfork changeup grip, thrown exactly like a fastball, and aimed to a low target (such as the catcher’s knee or top of his foot). If he doesn’t get any natural action, encourage him to start tinkering with the placement of his thumb, and/or placement of fingers on different seams, and/or some squeezing pressure between the index and ring fingers (it’s similar to forkball / split finger pressure). If he dedicates some time to experimentation, he’s bound to find some adjustment that causes the changeup to move and sink … and you can rest easily knowing he’s learning to change speeds and not hurting his arm.
It’s amazing how obsessive some kids can get when they discover that a changeup can do more than just go straight and slow. Present it as an out pitch and watch the excitement begin.