Yeah, I like where KC went with his opinion. If the pitch is ok on the arm, and he really likes the pitch, maybe limit to like 8 of them a game or a generally low number.
This way this will keep your son, coaches, or the catcher, from getting addicted to throwing the pitch.
I know a lot of guys 15 that haven't developed since they left little league, because the never learned to "pitch" they just through junk, and it doesn't work near as well in the older age group.
Just from scorekeeping the 13 year old only league, I can see how a curveball and the like can be detrimental to development.
Two kids that pitched the other night were straight junkballers.
Each about 5'11", a head taller than most kids their age. So you would expect lots of fastballs like the typical big kid pitching the first year on 60' where he still trys to throw it by him, but you would be wrong.
They each threw over 50% curveballs, straight junk. They domminated the 13's, giving 1 run up in 7, which is great because rarely do teams have less than 8 runs. But the kids have pitched in the 13-15 age group and have been schelled, because you can't hand a curveball to a kid that's seen his fair share of breaking stuff through the year.
On the contrary I was very successful at 13 in the 113-15 league, not throwing particularly hard for my age, but fastball and change, I had like a 2.5 ERA in 18 innnings, heck those numbers are better than what I had this year.
THINK IN LONGTERM.
Good luck to ya
PS: I say junkball when a kid is throwing mostly all breakers, and they aren't setting or serving a purpose really, they're just being thrown because they usually don't get hit consistently.
Plenty of the hardest throwers in MLB history never learned a curveball till they got drafted.