YES!
I'm thinking a combo of Asperger's and Reactive Attachment Disorder (RAD). He sounds quite a bit like my son. My husband used to tell me that he had no problems at all with-difficult child, so why did I?
"Because I tell him to do things. Homework. Chores. All you do is watch a movie with him."
And we live together in the same house!
You've got a lot working against you in regard to his history. I would focus most of my efforts on bonding, frankly, and setting parameters and house rules. If you have to, use the other autistic grandchild as your "excuse" for why things are they way they are. Of course, that could come back to bite you, if difficult child yells, "I hate having to do everything just because of difficult child 1!" and then difficult child 1 hears it.
I'm just throwing out ideas.
You have really got your hands full. I am glad that you took him off of Adderall because you need to see his real personality. There is plenty of time to start new medications, and it seems like that will happen soon enough.
Sigh. I am so sorry about the biomom. What a mess she made of his life. Poor kid.
You are a rock! He is lucky to have you and he will understand how lucky some day. Maybe in 50 yrs. 
