5 year old starting to whistle s sounds

Hi my very clever and articulate 5 year old son has recently just started to replace the s sound in words with a whistling "ssccchhh" sound. It seems to coincide with him putting on a baby voice. But the "sssccchhh" sound has started to come out in his normal speech.

We ask him to use his big boy voice when ever we hear the baby voice & have I spoken to him about the "sssccchhh" sound. But I don't want to keep correcting him as he is very sensitive & has gotten quite upset about it.

How do I help him lose this habit? Thanks.