How old is your son and is he actually cussing or is he just using bathroom humor? If he's actually cussing at home, in front of you or at you, then set your expectation about his use of such language, tell him what his punishment will be if he continues, and then firmly enforce it. When my older son was in about 2nd-3rd grade, the urge to say cuss words was just too strong, so I told him that if he wanted to cuss he had to go in the bathroom and close the door, because I didn't want to hear it. Well, that got boring after awhile so eventually he stopped.
If it's bathroom humor, well, I hate to tell you this, but this is SO TYPICAL for elementary school boys... (and beyond, really. My 13 year old sometimes still does the armpit fart thing.) The best thing to do is ignore it, honestly, because once he gets that you are annoyed by it, he will keep doing it. When my boys were little I did just ignore it, except if company was over or at the dinner table, then I expected them to stuff it! The good news is, that it is a phase, and does not last forever.