Thank you for answering my questions. That helps a lot & backs up what I was originally thinking. So, I wanted to give you this website to look at http://www.aafp.org/afp/2001/0415/p1579.html (you may have to copy and paste into your browser) if you haven't been directed to take a look at the symptoms of Conduct Disorder already.
I strongly suspect that this is what is happening with your child and that it may have been misdiagnosed due to his age. 8 years-old is young for conduct disordered behaviors to begin and a lot of clinicians won't recognize it or will "softball" it with a lighter (easier to hear) diagnosis. Additionally, he isn't exhibiting many of the symptoms yet, but it sounds like he is beginning to. Threatening, theft, and school suspension are hallmarks of this disorder. My guess is that, if not treated, the more serious violations of the rules will come later.
Conduct Disorder has a mixture of causes including genetics (which we don't know much about). Also, your child probably had Reactive Attachment Disorder beginning in his earliest months (the most critical time for attachment) due to his precarious home situation. Finally, he has ADHD, which is another risk factor for Conduct Disorder. Add the emotional stress of adoption to that and there is pretty strong evidence that this is what is happening.
Conduct Disorder is something that can be hard to treat, but its not impossible. First off, I would say that you need to find a psychologist (who works in conjunction with a psychiatrist, if possible) who specializes in treatment with children. Given his young age, the correct mode of therapy for him (regardless of his intellectual or verbal skills) is play therapy (by someone who knows what they're doing). It is extremely effective when done right. There will be family components that you can address and he may also need medication prescribed by a psychiatrist.
Anyhow, I will wait for your thoughts on this. There may be some components or information that I'm missing, so I don't want to go on and on. Please let me know.