His pain sounds like pleurisy to me.
Here is the best way that I can describe it in 9 year old terms (incase you want to describe to him). You have your lungs which are like a balloon, your lungs reside inside a sac -- like say putting the balloon in a sandwich baggie, if your lungs are irritated, infected, or damaged from acute or chronic injury the get inflamed. The inflamation causes them to be a bit too big for the sac they reside in and when you breathe the lungs ie water balloon touches the 'sac' -- thye can kind of stick. In my experience it hurts like hell sometimes so bad to where you cant breathe and cant move for a few minutes or longer and it can sometimes be a dull pain that lasts all day. Here is a nice adult appropriate description: <a target=_blank class=ftalternatingbarlinklarge href="http://www.healthline.com/adamcontent/pleurisy
">http://www.healthline.com/adamcontent/pleurisy
</a>
I have gotten up a few plugs but never had that sort of pain associated with them. Maybe mine just arent bad enough. For me this sort of pain is a predictor of bad things to come, generally a case of hemoptysis/infection.
oh and my plugs always remind me of a black jelly bean since they are generally very very dark-- but made of string, mine generally look like they have been wound up from one string of mucous over and over again.