Is he actually ready to transition, or are you doing it beause he's 2? If he's not ready, put him back into the crib.
Just something to think about...and take it with a grain of salt. I am merely commenting on something I'm seeing here and am only trying to help, not offend:
My ex's son was "abandoned" and neglected by his mother at age 2. Since she left, every unhealthy, naughty, difficult, or disconcerting behavior exhibited by this boy is blamed (by his father and grandparents) on his experience with his mother. "Oh, N is this way because his mom abandoned and neglected him 8 years ago!" This is ridiculous. The problem is that they're treating the boy like GLASS instead of treating him like a normal kid. When I met the child, he was the most spoiled little brat I've ever come across. I immediately instituted a consistent schedule, rules with consistent consequences, and treated him just like my boys. He was no longer "special" or "needy" because of what happened to him, he was loved just for being HIM, and if he was naughty...it was because he CHOSE to behave in a naughty way, not because of something that happened to him that he couldn't control. His behavior changed quickly and he became a much more enjoyable kid to be around. His father and grandparents saw this change and began to realize that by treating the child like he was damaged goods, THEY were setting him up for failure and lack of resiliency in every situation.
You've had your son since he was 4 weeks old. I highly, highly doubt that his separation anxiety is related to his bio-mom at this point. Nor do I think that he even remembers those unhealthy visits. It's more than likely that his separation anxiety is related to the fact that you seem to think that he is needing and above-normal level of care because of his past. He doesn't. He's not glass. He needs to be treated like any other kiddo who needs sleep training, so he can learn to sleep like any other kiddo. Lots of 2 year olds have a hard time transitioning. It's normal. Stop treating the kid like glass and he'll stop acting like he's made of glass.
♥
C. Lee
ETA: I'm sorry your baby had a rough start. I'm so glad to hear that he's with parents who understand him. It sounds like you're doing a great job and I'm sure that as time goes by, his separation anxiety will lessen.
Good luck with sleep training. Once he's ready to transition to a regular bed, I recommend putting a twin mattress on the floor in a corner with a bed-rail on the open side. Also consider a "bed tent" for twin sized beds to give him a nice, cozy feel.