I might have forgotten all I've read about your situation- forgive me if I have- but if he's been on an AD and he's having major behavior issues, I wonder why they haven't considered a mood stabilizer. My son started with depression shortly before his 11th b-day. Then he started having behavior issues that escalated into erratic behavior. They put him on prozac for a year. Then, after another erratic period, they stopped the prozac and eventually started him on mood stabilizers.
Anyway, I've been trying to identify triggers and think I have seen a couple- or at least it's very coincidental. But one very coincidental thing is that he seems to have a major growth spurt during or right after a "bad" period. Another, is that it seems to always occur in allergy season. Of course, these vary a lot from person to person and your son might not even have a serious mood disorder- it could be something else. I just mention it because I've had psychiatrists and tdocs say that boys particularly can start showing signs of mood disorders when the hormones kick in.
PS If he's having trouble with behavior right now, I don't blame you for not mentioning coming home for Christmas. It hurts, but we have to make parental decisions first.