5 weeks of coughing

Hi all, my poor little boy (2.5 years) has had his cough for about 5 weeks. He has been using his inhaler 2 puffs 4 x a day but it's not working, Took him back to the doctors today and he said his chest is still clear and nothing down his throat so try 4 puffs 3 x a day. Well as I type this he is really coughing upstairs in his bed and it's so distressing for all of us he usually starts to cry. The Ventolin is meant to relax the airways but I'm sure it's not working.
This is the third time he has had to use this and usually starts with a common cold.
Has anyone else gone through this? Is there anything I can do for my poor baby?

Debs xxxxxx