Oh no

Seb has started dropping the F bomb in the last week, I thought ignoring it would make it stop but since I've been ignoring him when he says it he goes "mummmyyyy" and when I respond he shouts "F$@%" it's god awful. It's also a tiny bit hilarious. There's still so many words he cannot pronounce to save his life but he can swear like a sailor.
He's also been saying shit. Like can pronounce the SH like nothing.

I shouldn't be too surprised, when J comes home from being away for 5 days on a dirty truckers work site, he brings home the potty mouth. Not that he's completely to blame, I need to watch what I say too.

So does anyone have any advice to nip this in the butt? I'll be horrified if he says it in public!