Can the stepmother get temporary custody of the stepson if his father my goes to jail?

My husband and I have been together for over 2 years. He gained custody of his 6 year old son 2 years ago. The child's mother gave up rights to the child to her mother and the grandmother did not pursue. Case closed. The child was under custody of the state. Until my husband requested custody and was granted it. The child has been raised I with me and my husband since then. I do everything for the well being for the child, as well as my husband. He has an excellent home, nurturing, we got him into school, teaching him right from wrong, and everything for the well being of the child. Me and my husband have 2 other children that reside in the home as well. They have been together since the child has come to live with us. I take him to appointments, school, buy clothes and nurture him, and help him with homework, anything he needs. My husband might have to go to jail for a few months to a year. Hes worried he will lose custody of his son and have to give him over to the child's mother. Who for the first year and a half had nothing to do with the child. No visits or anything. But was convicted by her mother to get visitations. The court granted supervised visitations 2 times a week for 2 hours. The mother has no job and has not had a job ever. She lives in her mothers basement. The house according to my husband and other witnesses is unlivable for a child. I am wondering how I can project the step son I love dearly and who I have taken care of for 2 years from going back to his bio mother when my husband goes to jail? By the way my step son calls me mom and loves me too. Please help me.