A 60 year old male ,no hx of smoking, alcohol,diabetes,HTN etc

A 60 year old male ,no hx of smoking, alcohol,diabetes,HTN etc. No asthma, no hx of allergy. One day while eating food, he aspirated some particles. That day he didn’t feel or think about it so much. After 1-2 weeks, he developed repeated cough with mucus overproduction. He went to many doctors and got treatment . While all were temporary tx. After 1-2 weeks of tx , he again developed cough with mucus production.

A) what do u think the next investigation must be ?
:sunglasses: what might be the disease ?
C) what imaging is needed to dx ?