I had one little guy like this. Same thing: watched questionable things at home and his previous sitter's. Older cousins and friends, etc.
I would tell him that we do not play/talk/act like that at my house. Commonly heard: But mommy lets me.
I would just tell him that that was fine but then he needed to save that for at home with mommy because there are different rules at my house. Redirected him to something more appropriate. It took months before I saw improvement. But I did see it.
One of the other things I did was upped our outdoor time in the morning. Let him burn off all his steam. That helped a lot.