Hi bmjsghowell,
Thank you for reaching out to us. I am sorry to hear your son is experiencing infrequent migraine attacks. It's not uncommon to have stomach upset and vomiting with an attack.
It sounds like you have a good idea of when he gets them, but I wonder if keeping a migraine diary would help determine what is triggering a migraine attack. That would be the first thing I would do. In fact when my son was 9 years old, we kept a detailed migraine diary for over three months to help determine his migraine triggers. We were able to figure out that his migraine attacks were triggered by dehydration, dark chocolate, interrupted sleep patterns and too many nitrates. Let me share information on how to keep a migraine diary;https://migraine.com/blog/keeping-migraine-diary-basics/ and you may want to take a look at this section on children/adolescents and migraine - https://migraine.com/migraines-in-children-and-teens/.
I hope this helps, keep me posted on how your son is doing,
Nancy