Right around the age most kids get into sex and drugs nowdays, my youngest brother was already getting in trouble with the law and was a father just a couple years later.
THERE IS NOTHING WRONG WITH WEIGHT TRAINING WHILE YOU ARE YOUNG. IT IS NOT WRONG TO WANT TO LIFT HEAVY OBJECTS REPEATEDLY.
Better that than fucking off doing something else that offers you no benefit. I see nothing wrong with learning dicipline, and the love for hard work!
I would say, thanks for the thought but he is my son, he enjoys it, and there is nothing wrong with it. Suck it and shut up.

Too young my ass, double those weights.
What I see wrong, is trying to persuade a kid to give up and not chase after something he or she enjoys that is completely harmless. No, thats ok son, put down that dumbell and go discover shrooms and pot with your friends. Go watch TV. But dont lift weights!