MY HUSBAND IS CONSTAntly Spitting Because He Feels There Is

MY HUSBAND IS CONSTAntly spitting because he feels there is something on the roof of his mouth and cheeks. He has been to the dentist, oral surgeon and family doctor and no one sees anything...this spitting is constant when he is awake.. I don't know where to go next...the spitting produces nothing, but if he doesn't spit, he ends up throwing up.