Hi
Its better to take him back to the doctor.
It might because he din`t drink or eat anything- he must be tired and sleepy.
If he has been prescribed some anti-histamines like
Benadryl too- he must be drowsy because of that.
Hope his breathing difficulty is better now.
But its better to see your Doctor again, get him examined fully and do accordingly.
Wish your son a speedy recovery.