Child swearing — help!

My 4yo spent the weekend with his grandparents and 17/19yo uncles. He came home Sunday night and has been swearing ever since. He fucking wants this, god dammit that, shit, bitch, he won’t stop now! What do I do to correct this?? I’ve already had a talk with my brothers and we tell him those are words for adults to say, put him in timeout, he doesn’t care.