Hi, Mom: Sorry to hear about your son's diagnosis. As others here have stated, though, it's good to know what you're dealing with so that you can take a proactive approach. Often I take comfort knowing I'm doing my best to fight this stuff.
I've had problems with sneezing myself, and that usually happens when I have inflammation in my spine. I would bet that it wouldn't just have to be the spine but probably costochondritis (sp?) also (I think you can find more info. about this on this website). I have heard of other people describe this pain, also. I didn't have problems with it for a long time, but when I finally did during a particularly bad flare, I had already read people's description of it so it didn't worry me as much. The breaks in between would almost seem to confirm this. Best to ask and check it out, though, just to be safe. For me, it seemed to be something in the forceful jarring of the sneeze that aggravated things and caused the additional pain.
I wish I had more to offer you than just my prayers and best wishes. I'm so glad that your son has you in his corner.