I think you did the right thing. Boys can be mean to each other but it is your job to teach them your family does not work that way and you are not joking. If he feels controlled, so be it. You are setting the standards of acceptable behavior. He can be free when he pays his own bills! God forbid he say something like that at school as he gets older. It will be a lot more serious than an investigation! The school may suspend, give detention, call police, order a psychiatric exam – they take these things very seriously since there has been violence at school. Once I got a call in first grade because my child made a shooting finger sign at another kid – they were playing cops and robbers! The schools have zero tolerance AND we live in a litigious society.
I would also explain from a softer perspective about love, respect and that family is forever. One day parents die. Siblings need each other and childhood can be so much more fun with a close relationship.