you are in an extremely stressful and dangerous situation.

His therapist is right in saying he can't be forced into a group home. HOWEVER....they can't force you to take him back into your home either. It's sort of like last call in a bar "we can't tell you where to go but you can't stay here."

You son is in denial, uses drugs, talks about weapons, and has a plan to commit an act of violence. Try to discount the fact that he is your child for a few minutes. Picture if a friend of yours was telling you that they were considering letting a mentally ill person who refuses to take meds, uses illicit drugs, and is violent..stay in their home. You would tell them they're crazy, and probably fear for their life.

No one can force your son to be treated. The single most helpful and loving thing you can do right now is refuse to help him remain ill and in denial. That's exactly what you are doing if you let him come back home and continue on as if nothing happened.