Dear  i can understand  ur situation n i feel very bad about ur kid ......but i can help u to improve  the condition a lot  and  he  can sleep without any trouble. Actully  the basic treatment for 
asthma is same as u r getting....but u can consult a chest physcian.
I  suggest u some 
home remedies which is so effective u cant imagine. 
In my opinion u should  provide steam  inhalation to ur kid before sleeping at nit
- stop curd , rice , bulky fatty meals especially at dinner.
- drink tea  n put  crushed ginger in boiling tea n add a pinch of black pepper in tea.
- Make a powder of equal parts of clove , black pepper , Tulsi / ocimum, and shunthi( dry ginger) and  lick i 1 gram of it with honey  3 times a day after meal.
- Add 1 gm of rock salt to 50 ml warm mustard oil  n mix it ... then 
massage with this warm oil  in whole chest of ur kid  before he sleeps....this will make him sleep comfortably n prevent cough
- use warm water for drinking whenever he  need it.
U can also  start some herbal medicines like
1) syrup Tulsi( himalaya drugs) 2 tbspn thrice a day after meal.
 Ask a chest physcian  to check the doses of 
bronchodilator drugs i n prescribtion as he feels choked due to 
bronchoconstriction.
Follow al these measure n see ur kid smiling .
Good luck