Soup, I'm not sure what you and the neurologist have already done together on behalf of your DS, but I can suggest asking, "under what circumstances would you order an EEG?"

When you say they are using MRIs to treat migraines, do you mean they are using MRIs to diagnose migraines?

If your DS had a brain tumor, he would likely show recent changes in speech, balance, vision, motor skills, or personality. If you haven't reported any sudden problems in those areas, then the neurologist would not be concerned about a brain tumor.

You probably already know a lot about melatonin:

http://www.medscape.com/viewarticle/496615

Neurologists have a reputation for being odd or cranky and hard to talk with. Maybe you already know that... My migraine-oriented neurologist does not treat me for insomnia and has made it clear she does not treat children. If you don't feel you are getting your questions answered, I'd definitely encourage getting a second opinion from a pediatric neurologist.