B.K.
If you haven't already, have his hearing tested. Tell him the correct way to say something, but do not keep on it more than once or twice, especially until you make sure that he can hear correctly. That can frustrate him and give him more serious issues, like self-esteem.
I come from raising a boy who needed tubes in / out in/ out, etc. of his ears (ages 3 - 11). That and speech classes were his saving grace. He is now an electrical engineer!