As far as I know, if you are the sole provider and he's physically lived with you since birth, you have \"custody\" of your son already, especially if you're listed as a single parent on any public assistance papers, medical papers, or legal papers. To be on the safe side, however, you can consult an attorney to find out how much it is to get \"emergency/temporary custody\" as some mothers do at the beginning of a divorce. Thay way, if he were to try and claim custody, you'll have actual papers stating otherwise. Remember though that even if you have custody, he can take off with your son if he's so inclined. It happens every day. So, if you ever get to the point of having genuine fear of this and know that it's not an idle threat, get a restraining order and explain this reason to the authorities.