Thanks everyone. He is 8, so I will work with him on watching for the signs. I was out with our oldest, so I didn't see when it started, just got the worst of it. He only tends to have them about 3 or 4 times a year, so I don't worry too much. They do tend to happen usually in the summer.
If it usually happens in the summer, I wonder if a flash of extremely bright light, caused by the bright summer sun, may trigger it? The blinding sun reflecting off the chrome bumper of a car going by can be stabbing, right to the back of the head, if it flashes in the eyes just right.
Some sidewalks are so light that at mid-day, they reflect light up in a blinding way.
One tip I have: I've had extreme headaches which turn to migraines, if not treated right away, since I was a child. NOTHING gets rid of the headache for me except Advil. I'd end up taking 3 aspirin as it never fully got rid of the headache. Even then, I'd still feel it lingering in the back of my head. Tylenol does nothing for me. Trying to sleep it off doesn't work. It has to be Advil (for me.)
Find what works for your DS and keep it on you and/or him ALL THE TIME. I have a little travel pill case I keep some in and have it in my handbag at all times. As, if I don't treat the headache right away, it will turn into a migraine. If DS doesn't carry a backpack, get those little portable, single dose, travel, envelope packets of the med, from 7-Eleven or the travel section of a drug store, and place them in his jacket pockets, so he knows they are always tucked in them.
I also always have some saltine crackers or bread around to eat before taking the pills, so I also don't end up with an upset stomach from the pills. If the migraine has gotten to a point where I'm nauseous, the only thing I can eat without the thought of vomiting is something bland like the crackers.
Last, tell DS to never go to bed with even a HINT of a headache. For me, if I go to bed with a slight headache, I wake up with a full blown migraine.
I have to take the Advil to get rid of the headache
before going to bed, even if it means dragging myself out of bed to take it.