How Should Nose Blockage And Coughing Due To Allergies Be Treated?

hi , my son aged 4 is having some allergy ..and he is taking cetrizine for 3 months then asthafen for 5 months now.. again now he has got severe nose block , coughing etc.. dr has given teczine..but WE Have taken allergy test for all common plant related and dust .. it all has come negative .. what to do now