No, I was referring to the heat sink–the type held in place on 2 sides by a u-shaped metal clamp, where you fit one side into a clip on the motherboard then have to push the other side in place (with a screwdriver) until it clips into place. And CPUs without integrated heat-spreaders, where you have a naked flip-chip die sitting on top of the chip package. I’ve always worried about accidentally cracking the chip die when clamping a heat sink in place.
What is the specific model of the tower computer? It would probably be good for you to also get familiar with the insides of that particular system so you can help him figure things out.
One thing that’s typical in most major manufacturer computers is that there aren’t any cards. All the necessary functions will be built into the motherboard. Connections for ethernet, video, and audio will often be part of the base motherboard. But typically on the motherboard there is the CPU, which will have a big, metal heatsink and maybe a fan. There will also be some small memory boards which look like long sticks of gum.
You’ll also see lots of cables plugged into the motherboard and to other components. The power supply is a big box, typically at the top/back side, with lots of cables coming off of it. There may be a DVD writer which opens in the front. There will also be a hard drive, which is a metal box about the size of a large index card. Before unplugging anything, look carefully at how things are hooked up and write it down. You’ll notice that the plugs will often have tabs or such so that they can only be plugged in one way. Be sure to keep that in mind.
Do not open the power supply. They have large capacitors which can discharge suddenly even if the power is unplugged.
Yes, modern motherboards are crazy integrated. I remember at one time having 5 expansion cards in my computer at one time–video card, sound card, SCSI card (a type of disc drive controller) SATA card (another type of disc drive controller) and modem. Now, you probably couldn’t even find a motherboard with that many expansion slots.
Here are a couple of pictures of motherboards (some of the first to come up on Google image search.) You can see that everything (including one or more monitor outputs) is built directly into the board.
Huh. I’m not familiar, or at least not by your description. Any system I’ve put an aftermarket heatsink into used a bracket that was pretty trivial to install. You had to slide it down the back of the motherboard, assuming you were doing this after the fact, but there was a hole in case where you needed to actually reach in and line it up. I’ll defer to your experience.
That said, watch them CPU clamps, people 
I think we might be getting ahead of ourselves. The OP’s son means something when he says that he wants to build computers, and what he means is probably not the whole process from silicon junctions to C++. Some parts of that process are likely to be fascinating to him, while other parts might bore him to tears. And professionals all specialize in one layer or another. Surely, the best approach is to first find out just what he’s most interested in, and then focus in on that?
+1
I was thinking the same Darren Garrison. Back in the day I always built my own box. Looking at your links to motherboards I honestly don’t know what most of the labeled components are or how to grade them for performance reasons. On the plus side I did see one connector on the Acer motherboard labeled as “Floppy Drive Connector”. 
I’ve been a computer technician for almost 20 years, and I’ve built well over a hundred systems from scratch over that timespan. The actual assembly isn’t rocket science. The first machines I built, I did so with zero knowledge or training. I literally opened an assembled PC and put the other PCs together by looking at the assembled one and copying what I saw. There are only 3 things that are relatively difficult.
- Getting every needed part.
- Getting the right parts (parts which work together and perform to your satisfaction afterward).
- Not breaking anything in the process (zapping with static, spilling something on your parts, using too much force and snapping something in half).
Even among those only the 2nd thing really needs some extensive knowledge, and that depends partially on how picky you are.