"Parenting from the Inside Out" by Daniel Seigel.
This book is amazing and helps you understand the role attachment plays in children's behavior issues. I am an MSW who specializes in attachment and trauma and the best tool for any parent (in my opinion) is this book or "The Developing Mind", also by Dan Seigel. These books help you learn how disruptive attachments with caregivers impact the developing mind, emotional regulation, behavioral symptomology, and offers idea's on how to ease anxiety related to insecure, avoidant, and disorganized attachment styles.
This poor little boy has been exposed to so much uncertainty and possibly even emotionally unsafe situations that is most likely impacting not just his behavior but his ability to regulate his emotions...and that classifies as trauma. His therapist SHOULD be able to talk with you about attachment and recommend some additional good resources for how to help him with achieving a secure attachment (which is ultimately what will help him resolve his anxiety and trauma) and the therapist should also be able to help you learn what protective factors are, as well as developmental assets. A quick google search of these terms might help you learn more as well.
You seem so very loving and understanding of this little boy, and gosh is he lucky to have you! There are so many children out there who don't have people like you. I'd be cautious to label him with anything, especially any additional diagnoses that could ensue and I'm glad his therapist didn't diagnose him with ADHD. Traumatic symptoms look very similar to ADHD and when medication is administered to children who do NOT have ADHD, behavior typically gets worse and the child suffers immensely with emotions that make them feel helpless.
I highly recommend "Parenting from the Inside Out"...you will learn more about him than you probably ever thought possible and you will also learn that with the right secure attachments and caring and supportive relationships, he will be able to progress to a healthy, non-anxious, and happy place with himself and others. Social interactions literally shape the biological architecture of our developing brains so poor attachments with primary caregivers wreak havoc. The good news? Positive social attachments with caregivers HEAL!
Lots of luck and as someone who works with trauma survivors, many in foster care, thank you so much for caring as much as you do. The world needs more people to care and do the day to day nurturing work with patience and persistence...I'm sure this little boy will thank you one day as well:) Whether you realize it or not, this little boy knows you care and as a result, you are already helping him both biologically (brain development) and psychologically.
Here's a link for more information if you are interested:
http://www.nctsn.org/nccts/nav.do?pid=ctr_aud_prnt_what