I think it's impossible for an outside observer to detect anything except low blood sugars -- when I was a kid, my parents could usually tell I was low when I fell asleep for no reason or if I started shaking. (I usually said (I still do, oops) I was fine, so they sort of had to watch my physical reactions to find out if there were a problem).
But, basically, you can feel a lot of different ways. Whenever I feel funny, I assume my blood sugars are either high or low. (So I test. Usually, I'm low, sometimes I'm high, and occasionally, I'm perfectly fine (which I totally don't understand, so I keep testing until I'm either high or low or I eat and have an excuse to give myself insulin).
Sometimes, when I'm low, I'm tired. Sometimes I'm really excited. One thing I've found when I get low -- for me at least -- is that my moods often seem to be an exaggerated version of whatever I was feeling before. So if I'm happy and then I get low, then I'm REALLY happy. If I'm sad, and then I get low, then I'm REALLY depressed. Same thing for irritability or any other emotions. (Happy, sad, and irritable are the only emotions I can think of. I don't think that's a good thing.) I get low a lot (I'm low now, so sorry if I appear to be rambling and oversharing -- I usually do that when I'm low), and I can feel all different sorts of ways. Shaky, tired, normal, weak -- it really varies.
I almost never let myself get high, but, when it happens, I get panicky (which is an emotional response to highs and happens to very few diabetics), tired, thirsty.
Basically, though, I think about the only way you're going to know your child's blood sugars is to test. A lot. I'm sure you don't want to, but until he's capable of talking and understanding, I think it's the only way for you to know.