That is the only proposal you can make. Your husband mans up and parents his child. Bear in mind, after the divorce, you may not have much of a relationship with your current stepchild.
You never know.
My brother has two stepsons from his first marriage, who he raised since they were babies. Their fathers were never around.
He's been divorced from his first wife for probably 14 years now, and the boys are both adults. He's always and still has a close relationship with them. The younger stepson came to live with him and his current wife, as a teenager. His ex gave him legal custody. That stepson still lives with them.
I know it's an unusual situation. But I also think it's wonderful that his ex didn't cut him out of his stepsons' lives after the divorce.
My brother's current wife has a former step mom who she considers her second mom, and has a maintained a close relationship with over the years, even though her father and the stepmother divorced when my sister in law was a teenager. The ex stepmother thinks of all the children/stepchildren in my brother and sister in law's family to be her grandchildren. They all go visit her out of state every year.
My nieces and nephews now have many adults in their lives who are surrogate parents and grandparents and love them despite the lack of a biological connection. I think it's a great thing.
I hope OP will be fortunate enough too, to be able to maintain a relationship with her stepson after her divorce.