Ain't Gonna Say Ain't No More

How do I get Tate to stop saying "ain't" all the time? Since reading "Shiloh" and "I Ain't Gonna Paint No More" (super cute book by the way- get it even if you are an adult!)- he will look for every opportunity to say "ain't". We talked tonight at dinner about it because I'm getting a little worried about him using that word at school and teachers wondering about his home environment. We talked about slang and proper speech, etc. He finally just said after this really long conversation, "Mom- I just ain't proper."