I thought hard about your announcement...and what to say. I guess, I'm sorry. It is so hard to get a child after it is too late. Yes, I think it was too late by the time you got him, although he was young. His infancy was ruined, he had no consistent caregiver, attachment disorder (sounds like the real deal) does not go away. The kids do not develop a conscience, which makes them dangerous.
I adopted an eleven year old who had severe attachment disorder, although we did not have that diagnosis for him when he arrived. We had to send him on his way for the sake of the rest of the family. He was destroying all of us. Hard to believe an boy so young could control us all and be as dangerous as ten adults, but he was. I understand why you are doing it and feel it is in everyone's best interests. This child should not be living with other children and may not even do well with two adults (he may need a lockup/Residential Treatment Center (RTC)).
I'm so sorry you have to make this difficult decision, but bet you feel like you have a brand new lease on life in about six months. And, in the end, most likely t his child will get worse and worse.
Big hugs to you and yours. Will be thinking of you.