I agree that it is likely there was a specific source for the anxiety--past or on-going events. The meds have a role but I'd prefer to have him talking to a mental health professional and hopefully figuring out what the real story is, then dealing with it on both a practical level if possible and helping him learn better coping mechanisms.
tg
I had school phobia too since I was 7.It started with my being terrified of certain nuns in the school I went to as they would use humiliation as a form of discipline (this was a long time ago)
We moved & I attended public school & had no problem until we moved again (military family) & the school phobia came back & I also developed a fear of people because I thot I was horribly ugly ( know as bdd or body dysmorphic disorder).I had many friends & was even in many plays, I told no one,parents or friends about how i felt as I was ashamed of it.I left school many times because I couldn't cope with being there.
Anyway, there may be an underlying reason your son is not sharing with you as to why he is gets so upset over school. I so hope he can get the help to get past this as alot of kids turn to substance abuse to cope once they're in HighSchool which of course makes matters worse.
I know this probably wasn't the answer you were looking for but having been there myself, I do know there could be an underlying reason & he is afraid or ashamed to share it with anyone including Drs.
best wishes,
Delores