I don't post a lot; mostly read. First and foremost, welcome to the forum. Have they done an MRI to confirm the AS diagnosis? Personally, I wouldn't accept a positive HLA B27 as a confirmation. The reason I say this is that some people can be HLA B27 positive and have nothing wrong with them and others will have problems, i.e., Crohn's, UC, AS, etc.
Both my husband and our oldest son are positive. My husband has AS and psoriatic arthritis, but by the time it was diagnosed (1983) he was already fused. Then again, back then they didn't have all the wonderful drugs they now have. The doctor's had both of our boys tested when they were five. The youngest was negative but our oldest was positive. It was just something to 'keep on the back burner'.
When the oldest was 15, he was diagnosed with Crohn's, which was recently changed to UC after the Prometheus blood test. He started having symptoms of AS when he was 17, but they weren't like hubby's symptoms. His would come and go, whereas hubbys would stay for a while. He recently had an 'attack' that just wouldn't go away, so they ordered the MRI. He had bilateral sacroiliitis and was starting to fuse. He was then sent to a rheumatologist who started him on Humira. He felt 100% better just after the first injection and hasn't had any problems at all with it.