I had undiagnosed joint pain as a child, a forerunner to my AS diagnosis. Apart from the no starch diet, I found that getting to the bottom of food intolerances was key. Possibly your son has developed certain food intolerances? Also consider whether certain sugars could be implicated. If they're not being correctly digested, they could cause problems similar to those caused by starch.
Spondyloarthropathy with symptoms of AS, Sjögrens and fibro. Previously took Celebrex, Methotrexate, Trepiline, Prednisone which seemed to trigger a flare and my illness became much worse. Symptoms now kept mostly under control with NSD, as well as cutting sugar, dairy and yeast and food intolerances. Mostly pain free now and all symptoms massively improved.