You do know it is going to happen if like me you take insulin to treat diabetes. You may feel confused and not quite right as mentioned above and need something to eat straight away, sit down if you can and test your blood sugar again in 20 minutes to see it rising.
I've been told I can be a bit irritable if I don't have something to eat at once.
Hypoglycemia, when not on insulin, is a different condition. The brain does need a certain amount of glucose every hour, (Google very helpful) and he may feel better and calm down after eating something sweet and sugary, such as cake, cereal with sugar, jam etc.