chronic cough and flem(phlegm)

Hi my father is 78, quit smoking 30 years back.
for the past 9 months, he has been getting very severe cough , and along with it comes the mucus
sometimes heavy , at times clear.

he went thru various treatments but is still not out of the problem.his chest is clear as per the CT scan and MRI, his blood count is very good btwn 11-13.

the cough intensifies when the food is spicy/oily etc or when he is physically and mentally exhausted.

could you pls let me know your views.

thank you very much