Chances are, your child does not even realise that the words he is using are considered foul language – it’s only your reaction, or those of others, that gives him this idea.
The best place to start is to stop using that kind of language around him at all, and to encourage everyone that has contact with him to do the same. If you do swear in your home, then explain to your child that some people don’t like that kind of language, and that it’s best not to use it when there are other people, like his preschool teacher around!
If you notice that your child is swearing to get a reaction out of you, then you need to control that reaction, and your emotions. Don’t get angry and don’t laugh. In fact, don’t react at all. That way, he won’t be able to distinguish which words are bad, and which aren’t, and they will lose their fascination for him.
Next, speak to your child’s father. Explain to him that his actions are hurting your child, and that he needs to be more responsible as a father. If he won’t listen to you, then seek family counselling. Even if only you and your child attend, it should help a little. Don’t let your child’s father do something that is potentially damaging without addressing the issue.