I'm so sorry to hear this. I haven't had this happen to any of my kids, but I have had chronic migraines myself. I go months without them sometimes, but then I'll get one that lasts 2 or 3 weeks and basically makes me feel like I can't do anything. I get a visual aura right before a migraine starts - I see white spots in front of my eyes, so I know it is coming. When that happens, I'll medicate up which helps some. I haven't found anything that really makes it go away though.
Major hugs to you in dealing with this with your son. That is the worst feeling, having your child in pain and not able to do anything to make it go away.