M.P.
I adopted my daughter at age 12. She came to live with me when she was 7. As an adult she is still dealing some with separation anxiety/abandonment issues. That's what it sounds like your son is dealing with. My daughter was older than your son and had suffered thru 5 foster homes before coming to live with me.
I suggest that your son will outgrow this. Two is an age during which babies suffer separation anxiety. Are you sure his reaction isn't normal for his age? It sounds like you thoughtfully dealt with the transition to your home. If he'd been with just the one foster placement, I suggest that he may not still be traumatized. It's good that he bonded with his foster family. That gave him a secure environment in which to learn trust. Given a loving and supportive environment we do heal from trauma.
You can check in with a child therapist who could evaluate him and help you better understand what is happening with him. One option would be to have him in play therapy. At 2 I suggest that is the best way to reach him.
I suggest that you continue reassuring him. When ever possible tell him ahead of time when there is going to be a change. Give him lots of cuddles/hugs as you enter the transition. And stop treating him as if he's traumatized. The way we treat children has a direct influence on the way they act.
It might help, if you haven't already done so, look up developmental stages in emotional growth. I suggest that at 2 he's once more visiting the place in which he continues to develop trust. As a baby baby he still saw himself as a part of the mother. Now, he's aware that he's a separate individual. So he learns to trust in a new way.