It sounds to me as though he most likely has some significant attachment trauma, due to bio mom's having been checked out on drugs, and having had multiple caregivers until 3 years ago. Bouncing a kid around in foster care is incredibly destructive to their ability to attach securely to other people (which is one of a multitude of reasons why our foster care system is in need of a significant overhaul).
His "disconnection" that looks as though he is "on another planet" may very well be dissociation, the "escape when there is no escape." Kids with emotionally unresponsive or physically neglectful mothers often begin dissociating fairly early, as a means of surviving the terror of encountering a mother who does not respond. Over time, dissociation will often become the child's primary means of coping. The child will then resort to it at any time when his or her world becomes too painful, frightening, or overwhelming to bear. And when a child has been through what your son has been through, that can be a good deal of the time.
When the experience of emotional and/or physical neglect is followed by being bounced around in foster care, it is nearly impossible for the child to come out without significant attachment trauma. He may very well have gotten the brunt of it, because he was the oldest, and the younger two may very well have relied on their big brother to be the stabilizing force in their lives (despite the fact that he was only a little boy himself). I would suggest that you find a counselor who specializes in working with attachment issues, and will work with you and your son TOGETHER.
As for the Asperger's diagnosis, I doubt that he has Asperger's. Kids with Asperger's usually do not want hugs, to the point that in some cases they find touch aversive. They are NOT inclined to ask teachers (or anyone else, for that matter) for hugs. Kids with attachment trauma, on the other hand, will frequently be nondiscriminatory in their attachments, sometimes even approaching total strangers for hugs.
Good luck to you. And bless you for giving these three boys some stability. They are very fortunate to have you.