Hi! I find tylenol to be very ineffective for my migraines as well. Excedrin is a wonderdrug for me. If it were me, I would take him to a neurologist that is a headache specialist. While I may agree that diet has some part to play in migraine prevention it is very difficult to pinpoint and if my doctor said 'just watch your diet' as his preferred treatment option I think I might throw myself off a bridge. Especially if your son is vomiting with his migraines- at least I don't get that with my headaches. I would definetely try another specialist who deals with headaches.
It's difficult enough being 12 and to deal with migraines on top of that....well....that would just be impossible some days! I wish him well!