Hi, Nathan use to do that occassionally, it lasted about a year. The doctor thought it was plueresia (don't think i am spelling it right, hopefully close enough for you to know what I am saying).
I would call the doc to be sure. He used to trap air between his lung and pleural area of the lung, and it is very painful. But since it is related to eating you need to make sure he doesn't have a blockage or a gall bladder issue. Which reminds me of the pain I would get after eating when I had my gall bladder attacks and had to have it removed. I know he is young, but it is my understanding that it is common for CF kids to have gall bladder problems. If it is his gall bladder, don't be surprised if the pain gets so bad after eating that he begins to feel like he can't breathe or catch his breathe. If it is his gall bladder he won't have a fever or nausea either, usually.
I would call the doctor for sure, maybe even page him tonight.
Hope this helps