Responding to OP, and have only skim read the thread.
Bananalover, FWIW I know what you're trying to say. Our youngest DS was a very difficult baby/toddler/young child. He would tantrum for hours, be extremely aggressive and violent. None of the discipline methods we tried worked (naughty step, removal of priviliges etc when old enough). One day, he repeatedly rammed his knees against his bedroom door for an hour, and his knee caps were black and blue. He was four. I had repeatedly been to the GP since he was a furious tiny baby, convinced that he might have been in pain. Nothing was ever found. He ruled our house for the first five years of his life. He hated change, and any tiny upset sent him in to a horrendous rages. Eventually, I contacted the school nurse and we were referred to a paediatrician. After a full assessment, she told us that he was exceptionally bright in some areas, but a normal 4 and a half year old in his emotional and physical development. This disparity caused him huge frustration. He could grasp concepts he wasn't emotionally mature enough to process, and melt downs followed. Once I had an explanation, I could work with him. We got in touch with Barnadoes, and they sent a worker 'round once a week. She was wonderful and helped us get out of the negative attention cycle (don't get me wrong, I know how hard that is when all you are getting is aggression and violence). DS2 is now 7, and he is a different child. He can still have melt downs, but now the triggers are easier to identify, and i know that there is a sweet, loving little boy behind the screaming accusations. Accepting that he has a challenging personality made all of our lives so much better, instead of trying to change part of his make up (he IS contrary, he DOES like confrontation, he IS stubborn). Now we focus on getting him to understand that elements of his personality need reigning in for his own sake.
This is a bit of a ramble, but I read your posts and I knew exactly where you were, IYSWIM. I've been on my knees and worried about the future for my small, furious son. You can turn it around. IME, sending a child like this away adds to the problem. DS2 is, at heart, an insecure, frightened little boy. I suspect your son might be the same. Has he been assessed?