How can I file legal documentation for primary custody for my son if I pass away?

The most important question for you is whether you already have orders concerning your son. If the father does not have order adjudicating him to be the father then he is not legally the father of your son. If he does have orders adjudicating him as the father, then he would be the next person in line if something happens to you unless he agrees otherwise. Some people in this situation also ask if the dad's parental rights can be terminated. In your case, if he has already been adjudicated as the father of your child, he is not likely to terminate his parental rights. Courts generally do not terminate parental rights unless there is going to be an adoption of the child. If you already have orders, the best option is to just ask the dad if he would agree that your child stay with your relatives if something happends to you. Then you should get a document drafted that appoints your relatives as guardians of the children in the case of your incapacity or death. That would be done through an estate planning attorney. If there are no orders, then the dad would have to prove his paternity if he wanted to assert parental rights if you died before him.