How old is he?
Is he being exposed to swearing in a way that you can limit, like movies?
You have probably taught and talked about why you don't want him to swear.
Does he have some good substitutes for cuss words? If he's doing it for attention, he might appreciate some comic alternatives. If he's wanting to express emotion maybe look for some more intelligent vocabulary for him.
Others will have more ideas, but those are my thoughts.