Migraines triggered by travel

Hi. My son studies in the US. He has always been prone to headaches. He used to get them after playing sport in the sun. He is 20 and yes he gets headaches on and off like most people but he seems to suffer from migraines always in May. He had exams in May and started to get a few more headaches than usual, then he took a long-haul flight to the Uk where he stayed for 2 nights and said he felt jet- lag and a bit headachy. Now he has flown to Tenerife and has been getting daily headaches. He wakes up and feels one coming on. I upland paracetamol don't seem to work. The same thing happiest year in May. He flew home and started getting these daily headaches. We wondered whether it could be allergies but no other symptoms and 3 different countries. We wondered whether it was migraines triggered by stress and travel. It seems like once they begin, he enters a cycle which lasts for several weeks. Can anyone shed any light on this for me?