Sorry to hear your little fellow is in such pain. My migraines started out at that age too. I'm teetering on the edge of one now so I'll try to keep this short. Keep him in bed in a cool, dark room, and keep the sound to a minimum. A cold compress on the forehead may also feel good. Give him whatever you usually give him when he runs a fever, maybe with some ginger ale, which is easy on a queasy stomach. If he doesn't show significant signs of feeling better within a few minutes of taking Imitrex, DO NOT GIVE IT TO HIM AGAIN!!!! I, personally, would rather have the migraine than the side effects of the Imitrex. He may not be able to verbally tell you if he is in that much pain.
If I were you, I would study up on migraines an be as knowledgeble as possible. Some doctors will just keep runing expensive tests because they don't know what else to do, and you don't want to end up in the poor house if it isn't necessary. You may need to see a Neurologist if your regular doctor can't come up with a satisfactory treatment. As far as the temperature goes, I often run a fever of about 101 degrees when I have a bad migraine. Just thought I'd let you know. Let me know if you have any questions.
Keep in touch and let us know how it goes.