Sorry to hear about his migraine issue and wish I had some solid answers.
Migraines can be caused by many things so its hard to nail it down to one specific thing.
Hormones? Maybe but it could be a combination of other things not related to his transition.
I take anti seizure meds and I never had migraines before being put on that medication.
I get migraines now but the Dr says its not related to the anti seizure meds. I think it is.
I know a few FtMs so I will ask them if the experienced anything like that.