Sound like this may be mostly an overarching inflammation issues.
Costo is a garbage diagnosis, and it is usually the result of an inflammatory condition. They have no idea why some develop this as a chronic condition. But for 3 days, while horribly painful, this may not result in a chronic condition.
Did he suffer any trauma in the area?
My gut is telling me that he is reacting to the celiac issue and that this will resolve. 3 days is not a long time for this as a "condition". You don't want him to have this chronically, and often the flair up of costo resolves in a matter of days to months, but for some it's chronic for years. It's when the pain continues past 4 months that it's considered chronic costochondritis.
Personally, I think it's rampant inflammation that settles in the rib area, and this is exerbated by bracing the shoulders and rib cage from the pain.
I hope he gets better soon. The pain from this is exquisite.