I was already pregnant when I met him and he signed the birth certificate. Now he's trying to take me to court for custody. How can I bring up to the court he's not his real father

Putting a name which you KNOW is incorrect on a birth certificate is, in fact, a crime. You obviously lied with the child was born. The Court is going to assume that whatever you say now is a lie.

The father will attempt to prove you are unfit. You will attempt to prove he is unfit. The Court will decide.