Claustrophobia
After the deep inspection, the father turned to look down at the son. "Alright, there are no monsters in this closet. Can you please put your clothes away now?"
The young son looked deeply at the father. "I… can't."
Desperately, the father asked. "But why?"
He stayed silent, couldn't tell his father why for fear that he may be called crazy. So, he just stood there with no answer coming to mind. The father, now knowing that there was no answer and no point in waiting for one just walked away from his son confused and yet frustrated at the same time. He knew that he wasn't disobeying on purpose; he knows that there is some kind of secret that his son wasn't telling him. Before he does, he uttered, "There's nothing in there, and I promise you that."
The boy looked deep into the closet. His blue eyes were wide and deep with concentration. It looked normal. But once the boy stepped in, he surely knew that the walls were about to cave in or worse- getting smaller suffocating him to death. Once he could get in, he knows that there could be a way not to get out. He knows that it's only a closet, and that there is a way out, but his fear being that there is no way out is what stops him dead in his tracks. So he made up the monster story, knowing that there is definitely no such thing as monsters but his father still buys the story.
He can't tell his father. He knows how much the frustration he had with his mother when she was alive. She had more of a tolerance for the closets than he did, but any other places that were small, she panicked. His father would think that he is crazy for having such a silly anxiety.
But deep down, it isn't silly at all.