I agree with Pika. I also use the Titan kettle for my solo trips, but have used it to do freezer-bag cooking for two people. Anything larger than that, I switch to the Titan 2-pot cookset (usually using only the larger 1.5 liter pot.) The 1.5 liter pot easily boils enough water for 4 meals; if the group is larger than 4, you simply cook in two shifts, since it only takes about 5 minutes for the second boil. If you want water for hot beverages, boil additional pots while the food is rehydrating in the bags; the beverages and food will all be ready about the same time. (I do this with the Kettle, which doubles as an oversized mug, eliminating the need for a separate cup.)
I like my MSR Pocket Rocket stove with the kettle or pot. With the kettle, if I'm using MSR-brand Isopro canisters, it's fairly stable (the MSR canister has a bit wider base than most cylinders.) However, with the bigger pot's larger diameter, I usually add an MSR Universal Canister Stand, which makes everything a lot more stable.
The Superfly stove is similar to the Pocket Rocket in weight, function, and fuel efficiency. The main difference is that the burner head is wider, which spreads the flame out. That's a problem using it with the Titan Kettle: the wider flame pattern tends to let the flame lap up the sides of the Kettle, wasting some of the heat. The PR keeps the flame focused on the center of the kettle's bottom. (If I were doing "real" cooking in the larger pot, I'd use the Superfly to provide more even heat.)
The Windpro is probably a bit overkill for just boiling water, unless your son is abnormally clumsy for his age (it's a very, very stable stove), or unless expected wind conditions are unusual and you feel a windscreen is absolutely necessary. (I've never found one absolutely necessary, except with an alcohol stove.)
The Pocket Rocket, Superfly, or Snow Peak stove Pika mentions would work quite well, I think; with a titanium pot (and you might want to look at the Snow Peak Trek Titanium series, which has some nice 1.5 liter pots) it will be the lightest option.
However, if weight isn't critical, you might also want to look at the various JetBoil offerings, too. I've used them, but find them a bit fussy and heavy for backpacking when I'm only boiling water and cooking in a bag. But, my buddy wouldn't trade his for anything, and gladly lugs the extra half pound or so.