I am so sorry your son was hurt! I assume you took him immediately to the doctor/hospital?
AS, or Ankylosing Spondylitis, is a chronic inflammatory disease that causes arthritis of the spine and hips, and can range in severity from mild to severe. We are not able to tell you what your son needs done, as there are too many unknown factors: severity of his AS, type of broken bone (exact location, severity of break), etc. Only his doctor will be able to diagnose and recommend treatment, based on his presenting conditions.
We can help you research your son's various options, based on his doctor's recommendations. Did you have specific questions?
You can read more about AS at: EmpowHER: AS.