Yes, this is most likely a factor. It's so hard to get him to drink enough already, so this heat is not helping! We resort to Gatorade a lot.
What a wonderful choice, Saint Joseph. Who better. Offering up Mass today for you both. You have a Great Faith, the open door to miracles.
Just read this PF - so sorry to hear about your son. I suffer with migraines, so I know about headaches. My pain is caused because of certain foods, and at times even because of dehydration. What may help is trying to convince him to start a certain diet that may keep him from getting headaches. It takes a little time for his chemistry to adapt to the diet. I think you can google the migraine diet. But I would consider taking him to a neurologist, and an allergist. Just to be on the safe side. Praying for you and your son.