My son Haruki say "Boku" in recent days.
"Boku" means myself, and it was said by self when he speak about himself as first word.
He used to say himself "Haru" or "Haruki".
What is his turning point?
I and my wife Ritsuko discuss it.
We concluded the reason was depend on his Japanese friends they are same age.
We had the good bye year party in my home on New Year's Eve.
At that time he met his Japanese friend, and he heard they said themselves "Ole" or "Boku".
So he followed them!