Idioms and Autism

Our son is currently working with his speech teacher to understand idioms. He isn't quite grasping the concept and viewing the phrases literally. This comes as no surprise, because those with autism have difficulty understanding idioms, sarcasm, or puns.
This morning I called to my son to give him instructions to get ready for school.
He didn't respond.
Not being able to go look for him I called louder.
He answered and said he was just in the next room.
Then I asked, "why didn't you respond the first time I called?"
To which his reply was,
"It went in one ear and out the other."

I laughed at his literal interpretation of the phrase. Still laughing.