First off, I am sorry to hear that you are having so many problems with him. I know it must be very frustrating and I truly wish you the best. I think that taking him to see a psychologist is a very good idea and will be the first step.
As for what to expect, my hunch is that this boy qualifies for a diagnosis of Conduct Disorder. Here is a link to the criteria for that disorder: http://www.behavenet.com/conduct-disorder
Conduct Disorder is very disruptive and can cause many problems at home and in other settings. If not treated it can lead to criminal behaviors that warrant prosecution. Children with a history of ADHD have a much higher incidence of Conduct Disorder.
Treatment for Conduct Disorder usually consists of very strict behavioral routines, family therapy, and medication for children who are also highly impulsive or aggressive (antidepressants and mood stabilizers are common). While mild and early-stage cases of conduct disorder may be effectively managed by family physicians, many children and adolescents with conduct disorder will require specialized mental health treatment. Typically, patients with conduct disorder are not distressed by their behavior; furthermore, there are almost always major family issues and dysfunctions that contribute to or limit treatment of the patient's problem. This is why family therapy is the treatment of choice. In addition to behavior management, effective therapy requires parental consistency, medication, and reduction of any family conflict.
I hope this helps - please let me know if you have any more questions.
1) Decrease any drastic changes in his care (if there are any). He should have 1-2 stable caretakers with no disruptions. Additionally, agreement about how he should be treated is key. He will try to exploit any disagreement between caretakers.
2) Institute a stable, reliable routine that he and the family sticks to. Dinner at a certain time, homework at a certain time, bedtime, etc. The more external control and reliability there is, the better.
3) Take away privileges for bad behavior, but don't punish or become punitive. If he sees good behavior he will model it in time.
4) Rewards for good behavior are also key.
- Unfortunately beyond that there isn't much that can be done. He most likely needs a good psychological evaluation and medication to see any real leaps forward. However, if you can stick to the aforementioned ideas it will be a great start!
Best of luck!