A divorce was granted to us in MI. We didn't list a son born as a result of an affair, only the two children that we conceived together during the marriage. How does this affect paternity, and what kind of trouble are we in because we didn't list this child? We were under the impression that we were supposed to only list the children that we conceived together, and now there is a paternity hearing between the bio dad of this child, and my ex, and myself. The divorce was done without the use of lawyers other than to file the papers for us.

Not sure about this but my best advice is to seek an attorney that could perhaps answer this on the phone.
Maybe someone will be here later that can give you a direct answer

I am assuming you were married when this child was conceived? But there is no support or custody order pertaining to this child now. It is questionable. Since you didn't list him, you basically admitted that you know he isn't yours. A judge could easily order a DNA test to determine paternity. You will need a lawyer if you want this child to remain in your life. As the father who raised him from birth until age (X) you may find that you have some legal standing. What position does your ex take?

I don't think you're in any kind of legal trouble per se. Since you mention a "bio dad" I presume that the child in question is yours but not your ex-husband's. However, if you were still married at the time of birth, depending on the laws of your state your ex may be legally presumed to be the father. If he in fact isn't, then the three of you will have to agree to a DNA test to prove that the other man is the father. Assuming that everything pans out accordingly, your ex will have no rights or responsibility where this child is concerned. He does, however, have rights and responsibilities towards the two children you had together during your marriage. When it comes to the son in question, you and the bio dad share the rights and responsibilities. WHoever doesn't get custody will have to pay child support and get visitation as per court order. The same will occur with you and your ex-husband regarding the other two children.