This is something you should have taken care of from the beginning. Only one state has ruled to set aside a child support order because the man was not the bio dad, and that is Maryland. But, it is being appealed to the US Supreme Court.
Once a child support order begins, you must pay it. As for setting aside what is owed, that is up to the judge, But, if she was ever on welfare, he is not allowed do that. Also, the state gets matching funds for enforcing child support, so a judge could say that it does not matter what the mother wants, the money belongs to the child.
Plus, there have been some recent rulings that when the mother does not go after the child support, when a child reaches age 18, he/she can sue for the 18 years worth. So, even with a decision now in your favor, it could be overturned in six years by the child.