All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First we confirm what we have mentioned in the previous Fatwa, that your marriage with this woman who was neither a Christian nor a Jew is invalid. You have to separate from her (and stop sleeping with her), however, your child is traced back to you as long as you married her assuming that the marriage with her was correct.
This woman has no right to foster the child because she is not a Muslim and is not from the people of the book. Religiously, you have the right in fostering your child, so you have to endeavour to find a way to take the child from his mother. If this has been made easy, then, all perfect praise be to Allaah, otherwise you can agree with her to visit him from time to time while advising and guiding him, and clarifying to him the Islamic creed and its excellence. Additionally, you have to do your best to treat him kindly so that he would love you and will be attracted to be raised on Islam principles and accept your advice until he reaches the time he would be independent from his mother. If you are unable to take your son away from her, or unable to visit him after doing your best, then we hope you are not sinful, as Allaah does not burden a soul more than it could bear.
Finally, it should be noted that a Muslim should not act before knowing the religious ruling about the matter in order to avoid unpleasant consequences, due to negligence in asking the scholars.
Allaah knows best.