You told him, you explained it to him in front of the whole class, yet he is not grasping it.  He needs the punch in the face to realize what he is doing.  You did your job, your concience is clear, so continue your teaching and do not allow a fool needing to show off bother you too much.

I have a student who is very fluent in English, mostly self taught until came to me to polish it off.  Her problem is that she did learn conversation by watching DVD of the show "FRIENDS".  So, she tries to copy their fast exchanges, and when she speaks in an effort to be fast, she misspronounces words a lot, and becomes very hard to understand.  She refuses to slow down a bit, claiming I do not know really how they speak in USA.  Well, I ended up frustrated with her, and told her not to return, I could not help her since she would not pay attention to what I would say. 
About a month ago she came back from a business trip to USA, and called me on the phone, and told me that she was frustrated.  That she thought she knew English, but persons back in USA had a hard time understanding what she was trying to say.  She was wondering if it was because of a slight accent, and asking me again what I thought.  I told her I knew what was wrong, but she did not want to accept that fact, so it was not worth going into it again.

It does amazes me to an extend that sometimes Chinese students feel they know more than their own teachers.

agree with HappyExPat. as a chinese i watch too many american tv series made me always want speak like act's tone. but too quickly to let other understand. some times even myself also don't know what i am talking about. so now i keep talk myself "slow down, you are not good speaker"

I had a similar problem! Only this guy was using terms like "chick" , "toots" and "sweetheart". I tried to tell him that those were no loner acceptable ways to address females in the west, but he said that I did not know slang because I was "too old" (I was 41 at the time).  Anyway, one day he was bragging how he had been chosen by his boss to host some customers from New Zeland. Then he disappeard.  Eventually, I found out that he was fired as the husbands complained that their wives were offended by him and took their business elsewhere. "There are none so blind as those who refuse to see'"

East Asian Arrogance again. Obviously 'they' know 'English' better than 'you' do. This is an emotional belief not based on fact. They have 5000 years of history, and don't need to listen to some 'foreigner' tell them 'how to learn English.'

Again, I am simply amazed at the type of people you run into in China. I've never had a student who thought he knew English better than me. However, that may be because all of my students are college age. Or maybe it's because I'm in a small town? Maybe a combination of both? There is one guy I know who has decent English who can get a little cocky, but he doesn't think he's better than me.

As for trying to explain it to him, you probably won't be able to. If he thinks he's better than you, he's not going to listen, and unless you can come up with a way to knock him down a few pegs, that won't change. Does he know anything about internet slang? Or can you compare the way you talk to your boss versus talking to your friend? Maybe you could use these as examples to show him that different situations call for different words and levels of formality. And also, try telling him that slang is only temporary and quickly becomes outdated. Tell him you're not impressed, ignore his "fancy language skills." And if all else fails, give up, because it's not worth it. Let him make a fool of himself, maybe he'll learn something.

Slang is no more than a generational code, or "secret language" that ties those members together. The problem that foreign students fail to realize is that it changes rapidly, and from one generation to the next, so the slang they learn today will more than likely be out of use by the time they visit a western country where they hope to use it. Not only will they not be understood, but they will sound plain silly (Borat comes to mind).

While T.V. shows and movies that feature a lot of slang sound trendy and "with it" at the time, ten years on, when viewed, sound hopelessly outdated, because in fact, that language dates the program in a specific time (Dy-no-mite!!).

When I teach my students, I purposely avoid slang, and only use it to point out the fluid nature of language. I tell them to learn the correct words, descriptions, expressions, etc., so that they will be able to communicate in any social environment or on any level. We go over both formal and informal conversational patterns, and when those are expected.

That being said, idioms are an accepted speech because they are rooted in culture, and not generation. The meaning of "you let the cat out of the bag" is understood by anyone who is past a certain age and is cross-generational. Terms like "heavy," "rad," or "tubular" are not. Language is too rich and varied to speak in trite terms, which describes "last weeks news," and "my main man."

As far as students who insist on using slang-heavy language contrary to the recommendations of their teachers, I have found that little can be done to correct the problem. Practical experience will take care of that in the form of laughter from those who encounter "all that jazz."

I think HappyExPat has addressed most of the classroom issues you mentioned, but I disagree with just one point: there IS a plethora of slang in Mandarin, and just like in English, some is more fit for teenagers, while other slang is enjoyed by the more mature generation. If you can read Chinese then just skim through blog posts and Sina Weibo tweets. There's a whole world of delicious euphemism out there in the Chinese language... make friends with some working class Chinese lads and you'll find it soon enough.