Just teach him to say "Fucking Ass!" for a while then offer the teacher the option to go back to "Oh My God".

But on a serious note, if the teacher is making your son upset and sad, you need to talk to the principal about it. Explain you are working with your son to break the habit and the teacher's behavior towards your sons needs to be more positive (reminding him of other phrases to use rather than scolding him).

In the meantime, you the parents might want to make an effort to try a new phrase. If your son stops hearing you guys say it, it will be easier for him to use something else as well.