I would really investigate about what your Step-son is telling you about being locked in a bathroom. He's 5 years old...and he must know what he is talking about. Based on his reaction...I doubt he is making up this story. He seems genuinely frightened and very scared.
Who was he with? Was is at school that this happened? Did someone play a prank on him? Did someone, God forbid, lock him in the bathroom on purpose? How did he end up in the bathroom anyway and locked in? Was it in someone's home or out in public? Ask the teacher at his school....see if she remembers anything about any incidents like this. There MUST be some truth to what your son is telling you....his reaction about is fear is very real.
I would be concerned. I know, well based on news etc., sometimes people lock kids in a room, on purpose. Would anyone in his life, his Mom, siblings (if he has any) or anyone associated with his family, do this to him? I'm just looking at all possibilities....
Try keep talking with him...see if he remembers anything else, or did he have nightmares as well? Associated with this, or perhaps a nightmare scared him?
Take care and all the best,
~Susan