I built a PC in late 2018. It took about 8 hours over two days. I never built a PC before; the most I had ever done was install more RAM into my laptop. Difficulty level was about a 7. I just took my time and tried to enjoy the process. You can find the parts I used for my PC in my Surly post history.
Paul's Hardware's build-a-PC YouTube series is good. I would also watch the guides by Bitwit, Jayztwocents, Hardware Canucks, LinusTechTips, and Tech Deals. They usually make a new video every year. It's good to see what each of them do; watching one video after the other will reinforce certain principles to help you remember important action items when you and your son start your build. If you need recommendations on parts, Tech Deals explains things very well.
Some miscellaneous advice I can remember from my experience:
If you want an easier way of connecting cables from your power supply, then buy a fully modular PSU. Connect the cables you need for your parts to the PSU first, then screw the PSU into the case, and then connect the cables to the components last.
My case is a mid-tower case with no optical drive and was marketed as a build-friendly case with a lot of flexibility, expandability, and modularity. You may not need as much with your son's first build, but it made everything so much easier for me to be able to move components around if I didn't like where it fit. It's not "see-through" in that there is no glass cover on the side to show off my parts. Buy a case with at least one dust filter. Better, buy one that comes with 2-3 fans. On the flip side, the cases that are more flexible, have better cooling features out of the factory, have more fans, have more connectivity ports, have a tempered glass side cover, are more expensive.
I stripped one of the screws for a large plate/cover inside the case so the plate is stuck and unremoveable. However, the case is spacious enough that I can still fit parts around it without any issue. At a minimum, buy a mid-tower case.
The CPU cooler was difficulty to install. I was extra careful when I installed the RAM because I kept bending the top half of the motherboard when I pressed down on the modules. The SATA ports were kind of hard to reach when I tried to connect the HDD and SSD after the motherboard was already mounted on. I dropped one of the screws into a crevice in the case and cut my finger trying to reach for it. You might run into these things on your build.
The expandability of the case has been so great. I've installed two more RAM modules without needing to remove the CPU cooler and another SSD. I plan to put another SSD and upgrade the fans.
Put the PC on a desk high off the ground.
If you are buying RAM that runs higher than 2666mhz, then check your BIOS to ensure you've enabled your RAM to operate at that speed (or check it in task manager). If your son will have a GPU (a video card), then plug the cable for your monitor into the GPU and not the CPU.