How To Treat Persistent Cough?

My 69 year old husband has had a persistent cough for about a year. He has had antibiotics, steroid sprays, nasal washes, two visits to the ENT department, where his throat was viewed by camera. We realise now that this cough started sometime around last November after a complicated tooth extraction. He was in the chair for 45 minutes, and the dentist was unable to extract the tooth. He was referred to another dental surgeon who successfully removed the tooth. Could there be a connection with the onset of this cough and the dental extraction.