Dr Lees said it was the first time he had experience of family members taking a child despite instructions they needed to stay for additional treatment.
"It's a serious illness and can be life threatening."
The boy had been improving at hospital after treatment but still required medical attention when he was taken, sparking the search, Dr Lees said.
By 5.30pm, police advised hospital staff the boy was taken to a local GP where he was treated and discharged to go home.
"That's the best outcome," Dr Lees said.
Respiratory physician and medical adviser to the Asthma Foundation Kyle Perrin said asthma attacks were rarely fatal.