SEVERE COUGH. It along with neublizer.

My 5 year old son (15kg )is getting cough frequently Atleast 3months once. During winter monthly once he will get and he will suffer for 3days very badly watever medicine we give it won't work for 3days from 4th day onwards it will start decreasing. Till now I have changed 4doctors but there is no improvement.doctors are telling seasonal allergic and it can't be stopped Doctor1: Asthalin Allegra Doctor 2: Monocef through it along with neublizer. 3months inhaler budecort Doctor 3: Singulair tablet for 3months. Cough stop syrup along with neubilizer Doctor 4: Motairkid Ascorill for 3days along with neublizer If not reduced from 4th day vistaclav for 5days Is there a way to stop this or Atleast reduce this. Please suggest.