Sudden speech change in preteen

[Shpeech]

My preteen boy has, over the last couple of months started changing the way he speaks. Instead of "just", he is saying "jushst", and instead of "history", saying "hishtory","shtop" for "stop".
When I draw his attention to it, he corrects it.
But it's difficult to understand him in normal conversation. He will add "sh" in to words everywhere. And I don't know why.
I think it started when he thought he was being cool, but I'm not really sure.
It's odd and when I hear him talking to people, like grandparents or other children, they have to really tune in to try to understand him or constantly ask him to repeat it.
Is there anyone with experience in this sort of thing who can tell me, does it stop, should I be correcting it?
He recently started secondary school if that's anyway related.

My DS started to talk with what he thought was a cool accent when he started secondary. He was told to park it at the pavement each day as it’s not acceptable in the home and around family and friends.

I don’t think it’s anything unusual. I sometimes catch DS, now 6th form, doing it, sort of talking with his mouth in an O shape instead of moving correctly. DD doesn’t do it at all, thankfully.

Thanks, that sound sort of similar. The lips making different shape is something we see too. He is over empathising a big o shape and teeth stay together, like his lips are too loose. It's very irritating and difficult to listen to!
Hopefully a phase, roll on the rest of the teenage years.

If you're sure it's deliberate then I would ask him to repeat himself every time. I can't stand stupid voices and people mumbling while not looking at who they're talking to. He needs to knock it off if he wants to succeed.

Absolutely agree. I have been asking him to repeat each time. It's deliberate but now becoming habit I think.
He also adds jushst to nearly every phrase.
I jushst need to finish my homework.
I jushst want to do it later.
When the phrase doesn't have a "just" in it. He adds it anyway, with the stupid sounds added!!