Reading your description was like a time-warp to me. I grew up in a house in which my brother was like this. He was a sweet kid, but had some serious issues. Firstly, I would take your son to another psychologist, one that is willing to really dedicate time to your child. I would also get your child tested for ADD or ADHD. Although it does not happen often, some children with AD(H)D act exactly as you are describing. Their brain cannot simply handle the rational thought of behaving. Dealing with your son is going to take a lot of patience and discipline, even if it seems he is not reacting to it. Another thing, and I'm sorry if this scares you further, because you really don't deserve that, but you need to hide anything that could be considered dangerous in your house. This includes knives, especially big ones that will catch his eye, pizza cutters (I've got a scar on my big toe from that one being thrown at me!), even down to the glass in picture frames. Also don't forget lighters and heavy decorations. Another concern is your daughter. Although she is not the problem child, she is effected by this at a higher rate than you are. My brother's misbehaving terrified me to the point of not trusting others with their anger. To this day, images of my brother's anger flash through my mind. She needs to talk to a counselor as well to make sure that your son's actions do not scar her. Also, when your son starts to behave better, it will be important to stress to both your children that it was a phase and it will not be held against him when the danger is passed. If not, your daughter will always have a fear of your son's anger and aggressiveness, which could cause this to be translated into later relationships. And your son might always hold on to the guilt of what he did. I really hope that you and your family can get though this.
20 years of personal experience.