Have you spoken to your DS and asked him what he wants? , how he feels?
I,m sorry you feel under attack by some of the posters but it is a very emotive subject, many of them feel a sense of urgency that you remove this boy from an adult who seems to have very little empathy.
It can sound meolodramatic when your living a daly life but these experiences for your child will have far reaching consequenses.
There are many older posters who have seen the damage inflicted on children, this will go on and on.
Maybe your son never liked him but couldn't answer back, maybe his bad behaviour is a sign that he's given up on you protecting him and believes he's on his own, and he,d better toughen up and fight back?
Who knows? but you won't know until your son feels totally secure and away from this family dynamic, maybe he'll never trust you again to open up.
His siblings could scapegoat him themselves as to court favour with their dad further adding to his isolation.
When your son is older, maybe married with children himself, it affects grandchildren who can be pushed away as they are not 'blood'.... your future DIL may resent you for not putting him first, it can be heart breaking.
You cannot possibly see the ramifications of your choices today, but I think people are gueinuinly concerned, he is alone and has no voice please put him first.
You, youself are concerned about this situation trust your senses, you posted this, there is no magic answer to make this relationship work.
Has your son got any grandparents that are close to him?
I hope so.
You sound like an understanding woman who is asking for advice but deep down you know the ultimate question your son is asking
Have you picked him above me?