C.M.
Have you talked to the boys parents about the problem? Maybe if it comes from them and you he will understand that it is not ok. You might also warn your other parents (if you haven't already) of the situation so that they can prepare their kids and maybe add some peer pressure as well. For instance, if the boy says it and one of the other boys tells him it is bad word that they don't say in their house, he will get the point. One other thing to think about is that now he knows that he will get attention when he says those words. Maybe try something to give him some attention when he says nice words and just ignore the bad words and then have a lesson time about using nice words that is directed at everyone and not just him. I hope this helps!