I’m terribly sorry you are dealing with such difficult issues. I hope you are getting your son and your family some therapy as well as legal help. This is a situation that has many, many layers to it. Your son needs treatment. You need support and help understanding how to be helpful. You and your husband need to find a way to work as a team through this crisis or you won’t have a marriage left.
It sounds to me like your husband hasn’t fully embraced your son as his own. From his point of view, he signed on to be a dad in a “normal” family. He didn’t understand that letting kids into our hearts means sometimes having our hearts broken. It may be that he can’t stand it so he’s distancing himself by making your son into an “other,” something that isn’t related to him. That’s at least one possible explanation. There are others, of course. It may be that your son’s behavior has triggered other painful memories, feelings, or thoughts in him that your husband can’t understand or grapple with.
All you can do is share with him that you need him and ask him to go to therapy with you in an attempt to salvage the marriage while you help your son. My heart goes out to all of you, as well as to victim and his family. This is incredibly painful for everyone.
I wish you well.