Why do children repeat what they have said in a whisper?
My four year old has recently started whispering back what he has just said...sometimes a phrase, sometimes a sentence. He is a bright child with a large vocabulary and very social. If you've ever seen the ABC show "The Middle"... that's what he's doing!
I don't feel that LouiseSattler addressed your question. It was not "why do children whisper", but why do they repeat what they just said in a whisper.
I can only speak from my experience of having done the exact same thing as a child. Now, at age 58, and having retrospectively analyzed my own behavior, I believe it was because of several factors:
1) I was socially ostracized in school (because of crossed-eyes) and being asthmatic (and so not being able to play like most kids).
2) I was a loner because of #1 and also because I was raised on a farm 10 miles from the city and so my social interactions were very limited.
3) I was ridiculed daily - both in school and at home.
Because of these factors, I was very self conscious of my appearance, my words - everything.
So, to your question, in my case, I was aware (even in childhood) that I was whispering back to myself what I had just said out loud because I wanted to review the conversation to see if I said it right or I had once again embarrassed myself by saying something stupid.
While I'm not saying your child has anything near my experience, there is still a possibility of him being overly self conscious and not wanting to sound "stupid" to others and suffering ridicule because of his choice of words. I would be asking myself is someone is picking on his choice of words or mocking him so that he is second guessing what comes out of his mouth?