(Not sure if this is common knowlege or not so don't think I am insulting you if it is) -Everyone produces mucus constantly. It is either used and disposed of in the stomach or "traped" in an area.
That said, my guess is somewhere along the way the mucus got stuck (although the fact that they didn't hear anything makes this unlikely) or he is fighting some small infection that is making the mucosa produce more mucus than usual, both of which will lead to increased cough to get the stuff up.
As long as he is getting it up is the important part, although if it continues you might want to get a culture done (as others here said before not all bacteria can always be found on the cultures).