AS is passed from mother to son. The most definitive blood test for you both is HLA-B27. 90% positive present with the disease.
It would be interesting if you think back to whether your father had back pain...
I acquired AS @ age 26 (klebsilla bacteria + genetic marker) *works pretty much like being born with a light switch & the environmental factor flips it on.
My 10 yo son complained of back pain while playing on the playground. I had x-rays & bloodwork done & he was diagnosed w/AS (HLA-B27+)
It is a shame your not near the best rheumatologist I've met in 26 years. Unfortunately, he is James Mossell in Tifton, Ga.
Good luck to you & your son.
P.S. I kept his AS in remission thru his teens with the antibiotic Keflex twice a day.

Married to a wonderful man 23yrs, w/ a terrific son diagnosed with AS.

Acquired AS 28yrs ago. Severe Fibromyalgia, Migraines, Multiple system involvement: heart, remaining kidney (no nsaid's allowed), spine (+ spinal stenosis), neck, knees (Rcd total Rht knee replacement 7/13, Lft scheduled for 9/13), thyroid, etc... Medically retired from US Military.
Now recently diagnosed w/ Primary Hypogammaglobulemia to boot!