What do I need to do to get permanent custody of my nephew?

My nephew's mother has already signed a power of attorney over to me and my husband but it ends at the first of the year. He wants us to get custody and his mom was trying to get to put him in a hospital for review. When I told him about it, he told her to leave him alone. That was in August. We have not heard from her and she will not return our calls. He can not go back to live with his mother because it was either him or her husband. She chose the husband.