Experience with Ankylosing Spondilitis Onset

Hi, My son who is 13 was recently diagnosed with having a gene for SA but Rheumy said it does not mean that he will get it.  However, he has been experiencing RA symptons in his hands and feet along with back pain and weakeness in his legs.  It appears that it maybe the onset of AS? From what I have read it's difficult to diagnose RA in children. the tests come back negative.  He has complained of backpain since he was about 5 years old and doctors said there was nothing wrong as he got older he was sent for physical therapy.  It seems that now as he is going thru puberty and the hormonal change may have set it off?   Also he is lactose intolerant and when he has certain foods that have dairy products or processed foods/junk food it sets it off and he has flare ups.  Can anyone share their experience with having AS and what it was like as the onset of it?

Thank you Blessings Rita