In Georgia, the age of consent is 16 years. So, a 15 year old would be a no no (unless they are married). Your son's father could indeed get in trouble. He could be prosecuted for statutory rape under Georgia law, OCGA 16-6-3. If he is still 20 years old, and she is still 15, this is a felony with a 1-20 year range. Once he is 21, and if she is still 15, this becomes a 10 year minimum, with a 20 year maximum. Now, you use the word "girlfriend". That does not strictly mean they are having sex. What I say above applies if it can be proven that they have had sexual intercourse. If they have done other sexual acts, the crime may be child molestation.
Neither you nor your son should get into any trouble. However, it may not reflect well upon you should you still allow your son to spend time with his father knowing what is going on over there. Terminating parental rights has a high hurdle associated with it. I am not advising that his rights will or will not be terminated based on these facts. You have suspicions which should lead you to seek the counsel of a family law attorney who also is familiar with criminal law. You should immediately take steps to address these concerns.
Call the police. If he has sex with the 15 year old, he is going to prison for statuatory rape and will be a registered sex offender for life. Even if he does not have sex, he may be guilty of other serious crimes. His behavior is not only sick, but a felony, and you will help get a child molester off the streets (and protect your child).
That police report and police investigation will also likely help document a civil case, for which you will hire a lawyer, to attempt to limit or change his visitation. You definitely need a lawyer.
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From the way you've written your post, it sounds as if the 20 year old father is the biological father but perhaps not the legal father? Even if he filed an acknowledgment of paternity and legitimation, he has no right to custody or parenting time with your child absent a judicial determination. Here is a relevant document published by the DHS: http://dhs.georgia.gov/sites/dcss.dhs.georgia.gov/files/imported/DHR-OCSE/DHR-OCSE_Paternity_Establishment/Paternity&LegitimationOfChildBenchCard-FINAL.pdf
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