He knows that he can really push your buttons with this kind of joking and you are continuing to let him know he does indeed have power in this regard. "Going off" on him when he does this will make matters worse. Tell him that he can joke around with his friends using these words (they'll soon tell him if he's being too gross or silly), but that he is not to use those words around you or other adults. You might also tell him if he really feels a need to say those words he can sit on a chair in the bathroom and say them behind a closed door.
You might also diffuse some of the tension and power games surrounding this issue by asking him questions about the words that he uses, why he thinks it's cool to say these words, when he thinks he'll be old enough so that he won't have to say them as often...you get the picture. If you remove the "goals" of this misbehavior -- seeing you lose it, getting attention, or getting back at you for something -- you remove his need to continue doing it.
