A lot of people here misunderstood the neurological assessment you mentioned, thought it was a neuropsychologist assessment. Is there a chance that you misheard your son's doctor? Could he have suggested a neuropsychologist assessment? It certainly is something to ask him.
As for your son - you feel there is a lot of anxiety there. It certainly would explain things. I know where adoption or fostering gets mentioned people often think, "attachment issues," but it's not necessarily so. Or there could be some level of attachment issue, but not the only thing or even the main thing by far. Don't stop looking for answers, is what I'm saying.
The level of violence you describe is still explainable by out of control panic and anxiety.
Your son has now definitely drawn the line in the sand and after he has calmed down you should be able to say to him, "We need to find out why this happens. We ads a family need to get some answers. You are an important part of this in finding answers for you as well. We want to help you be a happy, healthy, productive and capable person, living a good life on your own once you're an adult. But we have a lot of ground to cover in between and that is where it is our job to make sure you get any support needed along the way. Even if you don't think you need it."
Focussing on the end-game (for our kids, that's independent adulthood) might pull him up sort. Because at some level he must realise that how he behaves at home would not be acceptable at school or in a friend's home. If he can't learn self-control at home, then one day this monster will break loose in a friend's home or at school, and other people are far less forgiving.
I get that you love your son, but also realise there is something else wrong. But what?
If you have time before your appointment this evening (I'm not sure what time it is for you right now) go to
www.childbrain.com and run the online informal Pervasive Developmental Disorder (PDD) questionnaire on him. Regardless of the results, print it out and take it to the therapist. See what he/she says. It could help 'gel' ideas because it also puts together, on one sheet of paper, a range of different issues you may not have paid as much attention to. When you live like this, a lot of things begin to slide under the radar, because they become so commonplace.
Marg