If he signed an acknowledgment of paternity, he is presumed to be the legal father. He will need to file a motion to reopen the paternity judgment and this is not always very easy to do unless f the biological father can be found and adjudicated the father.
If he signed he is likely out of luck unless he can prove he was fraudulently induced to sign; otherwise, he only have a limited amount of time to get a test which has passed.
If he wants to see the child, he needs to file a motion for placement or visitation.
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I respectfully disagree. If you just found out that she is now saying it is not his, then you do have a change of circumstance. You can file in the paternity case for modification based on this. Unfortunately, you will need an attorney to get you through the paperwork required to do so. It is not likely that you will pull this off yourself.
Worse yet, if you know it now and do nothing you will soon lose your chance to file.