First, you have to make sure the kids are not exposed to that kind of language. I started really early and established the rule that such words were not used in our house. I'm very lucky I've got Aspies (youngest is Aspie-like) because this rule stuck with them and they pretty much don't break it.
DD1 was exposed to bad language when she was hanging out with the neighbor girl. Since she was exposed to it, it became part of DD1's vocabulary and eventually the words started to slip out of her mouth, even in front of me. So I put the kibbosh on that. I hadn't even realized that neighbor girl's mouth w2as so bad, but once I pointed it out to her, she stopped - it had become second nature.
Yeah, that's not very helpful, but I can tell you that recently I have been very out of sorts, and under additional stress. Those "bad words" have started escaping my mouth. I think the reason is because I am so stressed and I want to express my anger and frustration, and stress, but I don't have any brain power left to be creative, so I resort to this simple shock value language. BLEH Not pleased with myself at all. Even worse when it happens when I'm hollering at the kids for whatever reason. Yes, I apologize for my use of language.
Anyway, about a year ago DD1 started asking why we don't use certain words. Well, I explained that more often than not, the way the words are used is incorrect. For example, "Go fornicate yourself!" "You're such a female dog!" That one they are very confused about since they love dogs and don't understand why anyone would think that was a bad thing. "Go to the hot afterlife place evil ppl go to" "Homework just creates a vacuum!" None of these phrases really makes sense, so why say something that doesn't make sense?
Again, I'm lucky that I've got the type of Aspies that will follow this rule (won't follow the "dirty clothes go in the hamper" rule though) BUT your difficult child is only 11. There's still hope. Does he know what these words mean? You can institute a house rule where you don't use words if you don't know what they mean. If he continues to use them he'll have to (enter consequence). Stop him mid sentence and ask him to rephrase (I'm assuming the cussing isn't happening in a rage or tantrum). Do what a lot of workplaces do and create a cuss jar. It doesn't have to be money it can be points 5 points(cusses) and he has to scrub the toilets. Repeat what he says but with the linguistic phrases from above.
Whatever you do, try and do it soon, because it does become habit. I know I had a pretty bad potty mouth in 7-9 grades (don't remember exact) It was so bad, that my peers brought it to my attention. Once I was paying attention to it, I was able to curb it. ALSO, apparently FH and I were pretty loose with the lips when son was born. His third word was s**t. Oh what a proud moment! He said it while we were all in the car. FH just looked at each other and decided to cut those words out of our vocabulary.