I used to get Migraines but never that young, mine was always a pain behind the eyes, found it hard to concentrate and they made me quite Nauseated, often ending in me being sick.
I would suggest maybe just having a chat with your GP, is there any chance maybe it's more travel sickness? Being in a car or tractor for long periods of time might not agree with him.

Hope you get some better answers for your poor little man.