Thank you all for your replies. The patient does not have a family history of AS. He is gonig to see his doctor for an xray to check for spinal abnormalities.

I tried extension and it makes him worse. The longer he is extended, the worse the pain gets. His thoracic spine is very stiff, but lower cervical spine is very mobile. He definitely prefers to sit with posterior pelvic rotation and lumbar flexion as it is most comfortable.

A few of you mentioned the possibility of disc involvement. My question is: can it be an injured disc if there is an absence of neuro signs?