Since your son is still young, he has plenty of time to figure it out on his own. Hydration systems are kind of a personal thing, some people might like camelbaks, some like nalgene, that's why there are so many different types on the market.

When I was in scouts, we used metal canteens, and there weren't many other options at the time, so we just didn't know what we were missing. But as you mentioned, the drawbacks were the small opening so you couldn't add any powdered drink mixes to it (you'd never be able to clean it out afterwards). After a while, the wide mouth bottles came out (not nalgenes), then the nalgenes and camelbaks were coming onto the market. I've tried a few of different hydration systems like nalgenes and camelbaks, but currently I'm back to using the bottled water or large gatorade bottles. Durable, light, and best of all, free. Now my nalgenes and hydration bladders just sit around collecting dust.

I'm not a fan of the nalgenes because they're really heavy for their size, and for me I don't need a bulletproof waterbottle that can double as car chocks. You pay for all that durability in the form of extra weight. I'd rather sacrifice a little bit of strength to save on weight. I like the fact that some water filters screw directly onto them, but now they have adapters for regular water bottles as well, so that alone isn't enough of a selling point.

I never cared for hydration bladders because the tube is either in the way (when carrying a camera or other stuff), the mouthpiece always gets dirty when I set my bag down, and I like to know exactly how much water I have left when hiking. Plus cleaning and drying them is a bit more a pain. It's not difficult, just needs a little bit more care than rinsing out a water bottle. Also, I just don't like drinking from the mouthpiece, I don't know why, but even if there weren't any problems with them, I still wouldn't use them. They are nice for biking or other activities when you don't have your hands free, but other than that I still prefer bottles. Like I said, it's all personal preference, most everyone else I know loves them.

Sometimes ignorance really is bliss. When you're young, it doesn't really matter what you're using as long as it works. I know I never thought twice about the insulation in my clothes, the type of steel my knife was made from, what type of container was holding my water, or how many lumens my flashlight had. If only I could go back and not care, I would have saved a whole lot of money.