Advise us on stopping "slang" of 7 years old son.?

He is one of the most gentle boy in his cl as his teachers said so far. he is quiet and calm in school. He is good in all aspects in school. Its a great matter of worry for us, he started to use a slang in our own language which means stupid, bad, or something like that. Initially he used this only with us at home. We tried our best to stop him doing this and that, but failed. Now he also use in front of other peoples. If they say "NO dont do this. He says I dont care. If we say, NO dont do that again, he does more and more. We are in a very bad situation. Please suggest us with your thoughtful advise. We dont want to punish him. If we start to punish he will cry so loudly that will creat another problem. We are so upset with him now.