My DS has had to figure this out too. He is baffled by all the bad language he hears at school- he just doesn't get it. Regular, intentional conversations help. At this age, your son is starting to think more abstractly. At the same time, he's identifying with his peers. It's a good time to start asking questions that will get him thinking about who he wants to be as a person, and how his identify is separate from those around him. He can choose not to swear. He can choose how he wants to react. That's empowering. For us it's an ongoing conversation. The topics do get harder. Talking about swearing is great opportunity for discussion because, as PP noted, it's not really hurting anyone.