I too am concerned for the child involved. I am a police officer, so I have a different perspective. I don't think there is anything wrong with making contact with the police department in regards to the child's welfare. The worst case scenario is that the child is removed. Since his other siblings were removed, someone stepped in to protect them. I normally err on the side of not being involved, but when there is a safety concern for a child, an adult has to be that voice. My experience has been that the police would act if they know there is a child in the home.
Do you have any evidence that the child is still in their custody? The child may not even be living there. The grandma that is mad at you may already be providing care. I think you need to trust your gut, but maybe look into it a bit more before jumping in. If you are concerned that the child is witnessing the abuse, then reporting it would be helping the child. Others may disagree and that is ok. Someone has to be the voice for children.
All of that said, Your son has chosen this life. To detach would be, perhaps, giving him local resources for being a victim of domestic violence. The police may have already done that. A lot of people stay in abusive relationships because they are scared to leave and scared to lose their children. The girlfriend-she looks broken because she is facing the reality of consequences.
Your son will need to face consequences as well. As long as you take on his consequences-he has NONE. You suffer for all of HIS choices. You did not choose this life-he did. Not the mental illness, but he has resources available to help him. He chooses drugs (maybe not now, but has). The thing about our kids is this. We love them and allow them to mistreat us. We don't have too.
I would strongly urge you to take a step back and focus on yourself. He will need to learn self-sufficiency, as you will not be around forever. Who will carry him if you're not around? I suspect he will be on his own. Perhaps you can do the kindest thing and allow him to figure things out. There are social services for drugs, mental issues, violence and services for children. Offer him the tools and allow him to learn to thrive. The are mere suggestions, I am not a counselor and everyone's situation is unique.
He needs to grow and I hope you can step back in order for him to do it. I would love to see you do things for yourself and the rest of your family. They love you and need you as well. You are allowed to live and enjoy your life. You don't have to go down the rabbit hole to love him. I hope I have not caused offense. I don't want you to lose your family, I read that your husband threatened to leave.
I think you have given a lot of yourself, time and money to your son and his situation has not changed. No amount of love, money or support will change his life. He has to do that. He is 35 and has children, he is capable of change IF he wants it.
Please keep posting, I care about you and hope to see you take it easy on yourself. You are very kind an loving, I can see that,