Respected user, Hi I evaluated your query thoroughly.
*In my opinion this is due to  decreased lung immunity to fight against 
  external pollens .
*You require to take steps which give you slow but long lasting results by 
  enhancing his pulmonary immunity, which will not come by current medi.
@ Regular walking in fresh air to fill his lungs with adequate 
oxygen levels.
@ Deep breathing exercises along with YOGA 30 minutes daily without failure.
@ Keep him away from dusty atmosphere containing excess pollens ,
@ Try to make him wear Protective mask where there is more crowd where 
    he is prone to get immediate infections.
@ Avoid exposure to excess cold environments.
@ Avoid continuous exposure to air coolers / air conditioners.
@ Avoid excess cold drinks / beverages / chocolates / sweet items /biscuits
    consumption.
@ Give high protein & iron diet to boost up his immunity.
@ Tulsi with Turmeric are best Herbs for your kid on this earth.
@ Maintain chart of his weight , height & follow properly.
@ Maintain his 
anxiety episodes , give him more moral support.
@ Regular Spirometric exercise at home which is fun for him along with 
    definite treatment & definite measure to evaluate building up of his lung 
    capacity very easily.
@ 
Pediatrician consultation in case of severe 
cough / 
rhinitis / 
fever / any 
    discomfort in breath / decreased appetite / 
weight loss.
 Hope this clears your query.
 Thanks for using Health Care Magic & reviewing my answer.
 Wishing your child happy healthy life ahead.
 Regards.
 Bye Dear take care of him.