Hi. I can understand your situation and concern for your child. As of now, we cannot label him as Ankylosing spondylitis
based on your history and investigations, though he may fulfil the criteria for diagnosis at any point later in life. Moreover, a slight suggestion of sacroiliitis
is not a very definite report. Leg pain needs further clarification here, as pain is usually confined to the joints or entheses in these disorders. And pain is usually associated with early morning stiffness
lasting atleast 30 minutes, which is not so in your sons case. I would suggest that you visit a rheumatologist
who can decide on the further course for your son. He may only need NSAIDs on an as required basis, to keep himself symptom free at present. Thank you.