x-ray of SI joints and t/l junction will confirm this. Hla b27 is high in 90% of people with AS but is no way difinative. Was he negative for RF as well as this will help. Diagnosis a aero negative spondyloarthropathy. Is he adopted if not is there any genetic disease in family. Bilateral glut weakness can be due to pain of AS but might also be due to some other condition. I think AS is likely but it is a early presentation