I am sorry that you are having to deal with this right now, and esp that it is something you must deal with at this time of the year that has so many emotions and expectations attached to it. That makes it so very much more difficult. (((((hugs)))))
This is a really big issue, and not one you can handle in a day.
First off, why is difficult child only your problem? I realize you have had him since age 2, but husband is his father and you are not related by blood at all. I know family is made up of those we choose to love along with those who are related by the whim of biology, but in my opinion your husband is way, way WAY off base in expecting you to do all the work with difficult child and to bear all the blame. This is HIS SON, and he is the one who allowed him to be with someone with sub abuse problems and be shuffled through four other homes before he was 2.
this child almost certainly has Reactive Attachment Disorder (RAD). It is a condition developed by abuse and/or severe neglect during the first 3 yrs of life. The damage was done by the time you got him. He may not have clear memories of whta happened, or more likely is not capable of relaying them to you if he does remember them. He will forever be damaged by them anyway. he was taught that no one cared and adults are there to harm you and not help you and this means that he has NO ties to other people and he is SUPER dangerous to live with.
He NEEDS a very controlled environment, to not be around younger/smaller/weaker people, and attachment therapy. That is the ONLY kind of therapy he should have as normal therapy, the kind most people experience, will likely make him worse and he will manipulate the daylights out of most people, even (or especially) tdocs and psychiatrists. Read some of the books on Reactive Attachment Disorder (RAD) and how to handle it.
Where, exactly, does your husband expect to send difficult child? Unless you have adopted difficult child, you CANNOT place difficult child anywhere. Only his legal parent(s) can do that. I don't know how you would find a place, but to save your other kids you may have to set up a separate home for either you and the other kids or husband and difficult child.
Don't bet your kids safety on your ability to move/control difficult child. Even if you can at this time, in the next couple of years, four at the most probably, he will be at least stronger than you are, and in a rage you likely couldn't stop him. I had real trouble with Wiz because by age 9 he was stronger than I was.
CPS CAN take your other kids, but usually they won't. They threaten to, but legally if you have a safety plan and are doing all you can, the judge won't let them. Of course the judge won't get involved until it gets to court, which is often after cps has taken action.
While I know how scary this is, because I have been there done that, your best choice is STILL going to be to report this ASAP to cps. Beg for their HELP, for services, and say that you and the other kids are very afraid but if difficult child knows that then things get much worse so they won't tell. Then insist on a forensic interview with the children. This is an interview designed to get the child to tell what happened without leading them into anything that isn't true. People in general, but esp kids, will totally chagne their answers if you keep asking the same question. One study found that asking the child the same question as few as four times got teh child to not just change their answer but to BELIEVE what they changed the answer to is the truth. This is something that has been proved with many studies, not just one.
If you stay with your husband, PLEASE install cameras that record throughout the home, esp in the kids' rooms. Have them change in the bathroom, and make sure no cameras are there and that the kids are not in there with each other except if you are there (if you bathe the younger 2 together still). have a written safety plan that shows who does what, when and where wehn difficult child has a meltdown, etc... Include what you will do to make sure that difficult child doesn't molest the younger kids again.
I am NOT saying to leave your husband, just that if you cannot find a placement very soon for difficult child then you may need to have husband move out with im, or you leave iwth the other kids. As for no one to help, this is domestic violence and you can go to the local dv center and they should help. They may not have anything formal set up for this situation, but if you explain in an intake interview, and show a list of all the ways difficult child has hurt the other kids and you and any pets if he hurt them, the dv center should be able to do something or refer you to someone for help.
in my opinion your husband is TOTALLY out of line to make you the one responsible for difficult child's actions. Not only does it hurt difficult child, it hurts the family. He is also WAY out of line to say that difficult child should be left in his room alone all day. My heart aches for difficult child when I hear that his father wants that to happen! Can you IMAGINE how that must hurt difficult child? How it must reinforce his hatred of you and the other kids? Your husband is abusing difficult child wehn he insists that this happens. If I were you I would tell CPS that your husband is insisting that difficult child spend all day and night alone in his room. Yes, you do have to supervise all the kids at all times with difficult child there. BUT you cannot treat difficult child like an animal that must live in a cage. For that cps would probably either take ALL the kids or they would insist that you move out with your kids and take difficult child. If you refuse to take difficult child, cite that you are NOT his parent and you have no legal rights and cannot and will not assume any rights to difficult child because those are his father and mother's rights. They might send him back to his mom, or put him in foster care. or they might leave hm with husband with intense supervision. But cps WOULD have a huge problem wth a child not allowed to leave his room all day while his siblings laughed and played in the house. I know it may be the only real option to keep the kids safe, but it is still abusive and I think cps should know that your husband is insisting that you do this and that you don't think it is right but you need help to keep all of the kids safe in the home.
What type of person would keep a child locked away in his room whle his brothers get to play and have free range of the home? this is NOT a good solution and is NOT helathy for ANY of the kids. What happens to the younger 2 if they won't behave the way Daddy wants them to? If you and husband were divorced and you heard that he kept your sons locked away in their room, what woudl you do? how would you feel about it?
No matter how angry you are with difficult child, and you have EVERY right to be furious with him, and to be afraid of him, you still cannot condone abuse. You need to truly realize, deep down inside, what type of father/parent would lock way a young child every day in his own home. A father who would do this to one child will do this to the other kids if they don't do what he wants them to. This is a big red flag that something is wrong in your husband. in my opinion anyway.
I hope that you can find a way to resolve this and get the help that the entire family needs. I am sorry it all came out during the holidays.