When you say he is resting, how much rest is he getting? What is his course load at school like? Also, how much time is he spending on computers, smart phones, etc? He may need to take a few days and rest from everything, including anything electronic with a screen, preferably where he can take naps all day in a quiet, dark room.

Is he currently taking any of the vitamins that were mentioned in that sticky?

You mentioned that he is having problems with depression. Maybe it's time to consider talking to the doctor about an anti-depressant. Some people with migraines tend to manage them better with the use of an anti-depressant.

My heart goes out to him. It's hard enough to be 30 years old and trying to live life with this crap... I can't imagine going through this at 15. I work with high school students, so I see firsthand just how important their social lives are to them.

Edited to add: I just re-read your original post and saw where you said that the doctor told him not to go back to school until the headaches go away. Going to school with those headaches would only make them worse, but if your son is sitting at home and playing on the computer, watching tv, etc, all day, he may not be getting the rest that his brain needs. That's a hard thing for a 15-year-old to understand, and probably even harder for them to follow through on, but it may be worth talking to him and asking if he is really resting. Resting for the flu and resting for a brain injury are two different types of rest.