TLE, I can very much understand your problems with your DS - as I have some of the same with my own DS1, who is the same age.
My DS was also exposed to a dysfunctional relationship (with my Ex, his stepfather - my DS has never known his real dad). He never saw any violence, but he did see a lot of 'bad' behaviour - screaming, shouting, throwing things, real emotional abuse He also spent a lot of time with adults, as I have a large circle of friends, none of whom have children.
I can see now I have not imposed many rules on him; he has never had a fixed bedtime (& is up now chatting to his friends on msn), has always been up until all hours - this last week has been particularly bad, he has been up til midnight every night. He also is rude to me, answers back all the time and argues black is white. But only if I ask him to do anything.
For example, I have been asking him for the last hour to go and have a bath. I cannot physically force him, as he is a big, very strong boy for his age. Every time I prompt him, he refuses. This is because he is doing something he wants (ie on MSN) & therefore will not do what I want.
Other times if he's not doing anything, he is happy to help me, will tidy up, make beds, collect washing etc. So I end up not asking him to do anything if he is busy.....tiptoeing around his moods like I'm the child!
I never imposed rules on him because I didn't want to make him unhappy. The irony I suppose is that he is unhappy now probably because at least in part of the lack of rules. I am going to try and give him more boundaries going forward, and hope this works. However I am conscious that I can't change 11 years overnight.
In terms of your DS, I entirely appreciate his problems are more complex, & am not sure I can offer advice in my situation, but just wanted you to know that you are not alone