You intentionally married someone who hasn't been a large part of his child's life and you've decided to have children with him.
Your husband has been a deadbeat absentee parent who just recently decided to become involved in his sons life, and expecting his son to not have been traumatized by this.
His son is traumatized by him and other things and his internalized trauma and pent up aggression has externalized and manifested.
It's not a wonderful thing to see your voluntarily absentee father play house and have other children that he has been involved with their whole lives. Being forced to spend time with a man that wants to pretend like he's father of the years ...becoming only involved when he married, not before like he should have been.
He prioritized marriage and other kids over his already existing child.
Your husband left scars and traumas that will never be erased...and you being in the picture isn't helping.
His mother basically abandoned him, and let's him know that he isn't a priority like the other children..and that she "fits" him in whenever she feels like it.
Both parents have failed him, and you being there and having children with his father isn't helping the situation.
He feels alone, abandoned, unloved, and uncared for. He doesn't have stability, and everyone keeps passing him around and discarding him..
Plus he probably hears all or some of the terrible things you say about him.
You need to leave. How dare you expect your husband to choose you and his other children while excluding the son that he possibly is trying to make amends and repair his relationship with.
His son should remain with him inside the home for as long as needed. Unless your husband is planning on becoming a deadbeat parent again, he will be a parent for the remainder of his life. His son will a priority regardless of age or time.
You need to vacate and take your children with you. Your first priority should be ensuring that you and your children are in a safe environment, and safe.
You and your children aren't safe nor comfortable in the current situation. Do not give your husband an ultimatum.
Just leave.
That is also his child. Your husband didn't have his priorities straight and created a large portion of this mess.
He never should have been in any relationship nor have had anymore children.currently...until he worked on repairing the relationship with his existing child, as a single unattached man.
He and his son should have been living in the house alone together for many years and building a relationship...before introducing and including someone else to their lives.. Things may have been better for all.
This is all too much and too traumatic for his son to process.
Leave..let him and his son live alone and get the help they both need..perhaps in therapy...without you included in the picture for now.
Use this time to reflect on your own personal choices, including your choices in men and relationships.
A man who's chosen to not be involved in his child's life for most of it, should have been a major red flag to you and should have been a no starter relationship for you.
Hope things go well for all involved