Hi Mom Advocate!
You are VERY lucky...I say that because you have discovered your son's AS early. By the time I was finally diagnosed, my pelvis was fused all the way up through my lower 5 vertebrae as well as some between my ribs...
AS attacks the cartilidge between your ribs just as it does your discs in the back so that might explain some of the inflammation in your son's chest.
I only have one piece of advice. Look into ENBREL or any other TNF blocker. While anti-inflamatories will relive your sons pain, they will not stop the progressiosn of the disease. Even with exercise, and anti-inflammatories, he will still eventually end up fused:( After years of athletics and a ton of ibuprofen, it still did not stop the inevitable for me.
Enbrel will slow down or stop the progression of the disease. I am on it now and feel incredible but I will never be able to unfuse my body. Fortunately you have the opportunity to keep his disease from progressing and buy him some flexibility.
Those are my thoughts anyway. Good Luck!!!!