My three girls have all fainted at one time or another and I have received different responses from the medical professional. My youngest is a very tall 5'9 15-year-old. She grew very rapidly. She fainted in the Atlanta Airport in customs after we had been in Honduras on a missions trip. They ran a battery of tests on her including a spinal, but never found anything. It was very stressful as I am more for no intervention. But since she had been in Honduras it opened up a Pandora's box.
She fainted another time in the kitchen. The ambulance came . . .(you think I would have recognized it, but I didn't). She shook violently for a couple of seconds, said she felt funny and went out. Thank God I was there to catch her but in my panic said call 911. They gave her an IV. Said it was really common for tall teens and sent her home.
Another daughter, was trying to increase her water intake but hadn't eaten much that day and her electrolytes got out of balance. She also was put on IV (which really does work as far as hydration goes if you can stand the hassle of the emergency room). She received a severe lecture from a very judgemental nurse who thought she had an eating disorder. She didn't.
The third child, very tall and extremely thin passed out when getting into the shower. I was not there as she is in college. But she said she just felt dizzy and passed out.
My own intuition and knowledge tells me that they should cut out sugar, and caffeine and eat several small high protein snacks during the day to regulate their blood pressure. But do you think they listen?
Anyway, it is very scary but seems to be somewhat normal. It is great that your son has the awareness to feel it coming on. I would probably also consider his mineral intake. Does he eat enough real raw fruits and veggies to give him adequate minerals? And salt. If he is an athlete, he may need to eat a little more salt. Be sure it is sea salt.
I would start with the pediatrician instead of the neurologist unless he is also having headaches. Then I would go to a chiropractor.