I'm sorry you're going through difficulties with your son. It is a very tough situation to be in as a parent, because ultimately you want the best for your son, but you also want the best for his siblings, and you cannot and should not endure the abuse.
I think the best way to go is to get help, and that should be the ultimate goal. I know you mentioned that he refuses to seek help. Perhaps he might be okay with family counselling rather than one-on-one for him, which might make him feel less alone and as the one needing help, and more so that he is part of a family system that isn't working right.
I agree with Hayfa that there is an underlying problem that needs to be addressed, which may be giving rise to his behaviour. This is what a therapist will help with. You mentioned that he doesn't portray this behaviour outside of home. Have you asked why that is? Does he face similar problems with his friends? If so, why not? Is there something at home that triggers it?
Either way, it's important to take care of yourself, your wife and his other siblings too. Continue to have those conversations checking in on your wife and other children. If possible, continue checking in on your son to not completely cut off that relationship, giving further chance later down the line of amending it. Talk to him, and don't give up on getting help. Even if he is not ready, you and your wife might benefit from counsellors who help you two deal with your son.
Finally I think it's really important to make it clear to everyone, including your parents (his grandparents) and others getting involved, that this is not about blame. It's not about whose fault it is that your son is acting this way. It's about getting the best help as quickly is possible so that everyone is safe and so that your family system can function smoothly and happily.
Hope this helps. Thanks for sharing.