It certainly sounds like he is a good candidate for chiro, esp as teh medications ahven't helped. Just because one medication helps you doesn't mean it will help him. But I can guarantee there is a HUGE push against rxing fioricette or fiorinal to anyone for migraines, and esp to kids. You are far likelier to find a chiro to be of help than to find a doctor to rx those medications. (I just remember them helping you, am NOT saying that either you or difficult child have misused them, or htat you have given them to him.)
Are you working on keeping a journal of when he ahs teh headaches? Of the other factors that Star mentioned? One thing that can greatly increase headaches is preserved meats and aged cheeses. If you can just make a note of when he has bacon, hot dogs, ham, any smoked meats, etc.... you may find it to be a great help. After you have a month or so of journalling, go over it to see if there is ANY link between the headaches and weather, food, activity, stress, etc... I was VERY skeptical of htis until I was in a research study on migraines in college. I had YEARS of diaries that didn't show me anything. We ended up seeing a link - 2 or 3 DAYS after having more than 2-3 servings of preserved meats/aged cheeses I had a headache. Most people have more clear-cut links, but odd patterns like this do come up. It takes time and patience, but the increased use of pdas and computers makes it much easier to track this stuff AND to find patterns.
Do you think difficult child would be willing to just make a simple note in his journal, M for meat, C for cheese, ch for chocolate, kind of code, maybe with you keeping track of the weather, or have him download it from the weather channel each week? Then you could work on finding patterns in a month or two? There is a whole list of foods that can cause migraines, you can probably google them. It might make him feel like he had a bit more control over everything to do this.