i would have him tested for heartworm ESPECIALLy if hes going under anesthesia for teeth cleaning, the vet should reccomend it be done even if he had no symptoms especially before going under! goog luck keep us posted.

its not always a sysmptom that is constant, sometimes it flares up & sometimes not, also as the heartworm get more mature & larger the cough will become much worse & constant.