Regarding the story you told about your son having a backache...
1. Your writing: he had been carried to an emergency hospital My suggestion: He had been taken to emergency (no need to say "hospital") This is the natural collocation.
2. Your writing: And he couldn't know the cause for backache today too. My suggestion: And he couldn't know the cause for the backache today either. If the sentence is 'negative'.. ."I couldn't find out either" then, we should use "either". If the sentence is positive.. ."I could find out too" then, we should use "too".
3. Your writing: he slept hard in my car My suggestion: he slept soundly in my car This is the natural way to describe very deep sleep.