What Is The Cause And Treatment For Post Nasal Drip?

My husband who is 76, has constant problem with mucus in throat and cough at night mostly, to try clearing throat. Uses many tissues and has to cough and spit in them. His doctor does not know what it is, this has gone on for a long while. About 30 years ago he had sinus surgery for adnoids, this seems to have left more of a space where mucus travels and drains causing clogging and coughing. This has got noticeably worse in the past several years. It is not acid reflux and he doesn t smoke. He quit smoking in 1992. Lungs are clear.