Got sent home from school for threatening to stab someone

So my 8 year old step son just got sent home from school today for threatening to stab a student because he was "being annoying". He has severe anger issues, is on medication, and is in therapy. He is constantly in trouble at school for flipping desks, hitting, kicking, spitting, and he threatened to blow up the school. He has no consequences with either of his parents and it is really starting to get to me. He even hit our 9 month old daughter in the face and tried to lie about it when I caught him. My fiance didnt do anything about it and actually got upset with me for yelling at his kid.

It has gotten to the point where I dont know what to do. His parents just keep saying boys will be boys, but he is getting older and bigger and stronger. I'm worried he will hurt someone or be a bad example for my daughter.

Any thoughts on what I should do? Or CAN do rather?