plasticfork, there IS a nasty situation going on in your family, and you are now fully aware of it. What you have tried to do is resolve this situation as if you were negotiating with a normal, rational person.
Sadly you are not dealing with a normal rational person, you are dealing with an abuser. He is ENTITLED to treat all those around him as he sees fit, no matter who they are to him. This will never change. never. It'd be easier to strip women of the vote in this country than it would be to get him to give up his rights to cruelty towards those he wishes to hurt.
The fact that you are defending and protecting DS is good, but nothing can stop DS seeing WHY you are springing to his defence, he probably appreciates the kindness from you, but we all know it ought not to be happening in the first place.
we all know, you included, what you need to do, and you need to do it as soon as you possibly can.
Don't panic, plan it properly, but be absolutely laser-guided clinical with your need to amputate this gammy limb. don't deviate, don't allow yourself to be detoured, be intransigent. Think of your DS every single second until that man is out of your home/life. His behaviour is unforgivable.