Seems kind of soon to start aza. IMO. Usually if a flare persists after prednisone treatment, or the patient, like myself can't get off the pred. without flaring up, the DR. will go to aza.
I've been on it since May, and it seemed to work pretty good, though this past week I had a setback when down to 2.5 mg pred and am back up to 7.5. Started at 75 mg aza. and now at 125. Fatigue is pretty bad for me, but other than that all I'd say is that it takes a while to kick in.
I'd also add that like your son, I was pretty fine when first diagnosed. Just occassional blood, but no diarrhea. I was put on Colazal and 2 weeks later started a major flare. But I don't know what it was, as I've had no problems really with Asacol or Lialda (though I don't think either do anything for me.)
Dx Left-sided UC Dec. 2006
Azathioprine (Imuran) 125mg/day
Prednisone 2.5mg/day (trying to taper-started at 60 6/1)
Multivitamin, B complex vit., fish oil, calcium, fiber supplement, probiotic