As a person who grew up not knowing I had reactive hypoglycemia, I think I might be able to tell you a little bit about what it feels like.
I can go from being completely fine and not hungry at all to being so ravenous I might die in a period of five minutes. I never realized this was not normal until I read an intuitive eating book called "Are You Hungry?" and realized that I never had the one to two hour warning period of gradual increases in hunger signals that most people get. When I talked to my siblings, they agreed that they were exactly the same way -- not hungry to ravenous in less than five minutes. And when I say ravenous, what I mean is it literally feels like I will die if I don't immediately get food. A pounding, strong headache, desperation, intense irritability about whatever I'm working on. It's nothing like the gradual sensations of hunger I've read about.
The only way I've found to avoid it is to eat lots of small snacks irrespective of whether I'm "hungry" -- my goal is 6 per day, and I have a schedule where I know that if it's 9:30,I'd better eat something or by ten I will feel terrible.
So when you say that it's hard to get your son to recognize that he's hungry -- maybe it's because he doesn't feel hunger in the same way that you do. I didn't realize until I was over 30 that most people do not recognize "hunger" by an intense headache. But eating something ALWAYS made my headache disappear.
I don't have children, but I would venture to guess that the best solution for children is to feed them very often (and always small meals low in carbs and sugar) until they're old enough to understand what hypoglycemia means and can monitor themselves by just knowing that 7, 9:30, 11:30, 1:30, 3:30, and 6:00 mean that it's snack time, no matter what, and regardless of whether they want to interrupt what they're doing to eat.
