I had hive really badly and they were stress related. Once I changed some things in my life they went away.
At the time I was working 4am to 12:30pm plus a part time job so I was not sleeping. I was also in process of purchasing my first home by myself. When I had an anxiety attack (I really thought it was a heart attack, same symptoms) that put me in the hospital that's when I had to slow down and they went away on their own.
You say they start in the fall, my guess is that maybe something is going on in school that is really stressing him out. Could also be lack of rest if he gets up early and does a lot of extra activities.
Added: I took Claritin for hives, but I don't think they make it anymore. It was the ONLY thing that helped give some relief. Ask the pharmacist if they have it in prescription form and/or talk to the doctor.