Conflate nebbishness with maturity.
Call self-amusement thoughtlessness.
Your child is in high spirits? Tell him to quit farting around and take life seriously.
Only compliment your son after he’s earned it through self-deprecation.
The idea is to condition him, like Pavlov’s dog, to expect vitriolic reproof should he ever dare to rescue his mind from the quicksand of self-doubt. This is for his own good. It’s hard to get into trouble when you’re afraid of your own shadow.