I hate to say it, but unless step-father has legally adopted your son, he has no legal rights with regard to him. Since biological father is still around, SF couldn't adopt son without him terminating his parental rights and agreeing to the adoption.
Because the father still retains his parental rights (ie: he didn't give them up in order to allow the step-dad to adopt the children), father is the sole remaining legal guardian for the child if something happens to you. Even though the child has been living with him, the step-dad has no legal relationship with him.
If step-dad feels strongly, he may petition the court for visitation rights. Many states have provisions to allow those who "have an interest in the welfare of the child" to be granted visitation, including grand-parents and step-parents. As such, if the step-parent can demonstrate that he has developed a parent-like relationship with the child, I would expect to get a fair hearing from the court and consideration of the request.
The child's own preferences in the matter is something that the court will consider when making decisions.
If the step-father believes very strongly that it would be in the best interest of the child for him to be the guardian and for him to have primary placement and custody, and that the father is unfit to have the child stay there, then the step-father would need to consult with a ND family law attorney to find out how strong of a case for unfitness they feel they would need to present in order to overcome the legal reluctance to interfere with a parent's right to custody of their own child.
So the short answer is that there is not anything you can do without ex's agreement.
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