I will give a recent example as to my thought process of protecting your son's arm health.
My son pitched 3 innings to get the save on a Tuesday, 66 P.C. He had one inning where he just couldn't find the strike zone.
Our team is in the process of making up two district games so Wednesday we play out of town and after the 5th inning I see my son warming up. I see the he and the coach talking when he takes the mound in the 6th and evidently my son says he was OK with just one inning. He threw 22 pitches that inning. I could tell from my radar gun that he was down 3 mph cruising velocity.
Personally I would have rather my son not see the mound for 3 days but my goal is to get my son to stand up for himself as I may not be there.
We play again today so if he was your son would you tell him to tell the coach he is not good for today? I will.
I realize that the coach gets caught up in the game. I also realize he doesn't keep track of P.C. So it's up to my son and I to look out for himself.
1. Your HC doesn't keep track of pitch counts? That's just lazy and irresponsible.
2. You bring your own radar gun? That's a little over the top.
Well if it matters I use the radar gun on all pitchers. That way when I see an opposing pitcher throwing 70 mph I can tell my son on the batting circle to be very patient and scoot up toward the pitcher. I also can identify when fatigue sets in with my son.
It's real embarrassing when I yell "Scoot up toward the pitcher" in a game. 
It's also real fun when I'm wearing a D1 University baseball cap and have my gun out.
I gotta try decking out in mlb garb. I'll bring my stop watch and a clipboard and get me one of those floppy fishing hats.