Tents these days are much quicker to put up (even the non-instant ones) than they were in years past. Clipping the tent to the poles is much easier and quicker than snaking the poles through a long sleeve. Besides, wouldn't your son will be primarily responsible for setting up and tearing down the tent he sleeps in? A couple extra minutes won't be the end of the world.
If you'll be doing a lot of tenting for many years, undoubtedly a good quality tent makes a lot of sense. If you're talking about a few weeks for a few years, I personally would probably go for an inexpensive tent and toss it when it finally ceases to be useful. You can buy a whole lot of $75 tents for the cost of a really good tent. (Similarly, I'm not afraid to get a Harbor Freight tool if I'll likely only need it a couple of times, rather than jumping for a Snap-On tool that is much better and much more expensive....)
Definitely get a rain fly. Definitely verify that your campgrounds will permit a tent on the site--many do, and many don't. Definitely look for one that's tall enough to stand up in.