Stepsons mother should consult with an attorney if she's interested in contesting this.
This answer is for general advice and does not create an attorney client relationship with James Mosteller or the Mosteller Law Firm LLC.
You have my sympathy. It's tough to be a grandparent in the middle of a custody problem.
You referred to Tennessee and Florida in your statement. It was unclear whether any court had ever entered an order regarding the child. Depending on what state entered that order, you should consult with a child custody lawyer in South Carolina, Tennessee, or Florida. From that starting point, your attorney will be able to advise you whether that state still has jurisdiction over the child and whether you may have any rights.
You have my best wishes.