Could be bipolar, but I'd bet on attachment. From what you said, it seems your son knows what he needs to do and how to push your buttons. I think you're making th right decision not to let him move in with you. As heartbreaking as it is, you cannot help someone until they are ready to help themselves.
www.radzebra.org should have a list of attachment disorder symtoms. Unfortunately, there doesn't seem to be a lot of treetment or even research on treeting adults with this disorder.
You are not responsible for this. You made the best decision you could at the time and you had no way to know what would happen with the adoptive parents.
If he's saying nasty things to you, stay calm and tell him you're sorry he feels that way, but do not let him verbally abuse you(that will make him sicker and it won't do you any good either)
I wish I had some more hopeful advice for you.