J.L.
I know you may feel like its regressing- but, I would make his bed back into a crib. My son did that same exact thing. It was torture. To all of us. Now I look back and realize he was insecure with that side off. He loved his crib and I should've left well enough alone. We put it back on and the first few nights werent so good either. I think he was still confused. I let him pick out a new,small stuffed animal & told him it wanted to sleep w/him in his crib. Then kept telling him when he gets bigger- he'll get a big- boy bed. He stayed in his crib til he was over 3! He welcomed bedtime and we all had sleep! When he did get a big bed- we got a bunk bed and he loved it. Maybe it was timing- maybe it was a secure feeling type bed. Don't know- but I do feel like I was rushing him out of his crib before. Good luck. I wish my son was in the crib again. He's now in college!
J.