Teams throw curves at young ages because they discover that it helps them win. If winning is what you're after, by all means, put your son's arm at risk and let 'er rip.
When your kid is 15, though, no one will care who he beat when he was 11. Just thought you'd like to know.
People WILL care if he developed his fastball's velocity and command. And they will care if he stayed healthy or if he has a history of arm trouble.
You'll see a bunch of curves at the Little League World Series this August, just like every year. For most of those kids, that's the peak of their baseball careers, so I can't begrudge them trying to win it all when they have their one shot at glory.
But if you're not one of those handful of kids, and you're just playing your standard travel tourneys, what are you doing it for? If it's for development of your son's talent for the future, then throwing all those curves right now is destroying the future you claim to be trying to build.
I can't tell you how many bragging dads I used to hear talking about their stud son and all his K's when the boy was 11, who now wonder why he can't throw 80 in high school. The coach is a jerk, he doesn't appreciate him, it's all politics, yada yada yada. Lots of excuses, no accountability.
If you're serious about a future in baseball, prohibit your son from throwing curves until he's at least 14, and insist that he be removed from games if you catch him doing it anyway.