Hey Now Fubar!
I took a look at what your son is looking at and input the items into one of my favorite sites:
https://pcpartpicker.com/ in order to see if there may be any hardware conflicts as well as check on pricing.
You stated in your first post that the budget is $400, however what he is looking at is going to run over $800 to get what he wants as well as getting things he did not account for (cpu cooler, OS, thermal paste) also, he may or may not need an optical drive depending on how he plans to install the OS onto the drive. Now, I did not see any hardware issues, but as of right now, the site states that this build (I did add the cpu cooler (picked for being well reviewed as well as being pretty silent), Win10 Home, and thermal paste) and it sits at: $886.34.
Now, there are other options you CAN look at.
I usually watch a LOT of Youtube and one of my favorites is Linus Tech Tips, this vid covers budget gaming PCs with one build being under $300 and another ~$500. I would recommend the $500 build for the better performance as well as the SSD.
https://www.youtube.com/watch?v=Lfcschg9uwUAlso, there are other youtubers with their own budget PC builds and I recommend you look at those as well as it may give you some ideas.
Lastly, if you do NOT want to build a PC, then I would recommend woot.com or amazon.com. Frequently, Woot.com will have gaming PCs for sale from pre-built suppliers such as ibuypower or cyberpowerpc. This is the route I went with my boys (19 and 14). I waited for a decent deal to show up on amazon, my budget was ~$1000 for their PC, and ended up getting a decent system for ~$700, which I then upgraded with better GPU and PSU. About 1 year later I doubled the memory from 8 GB to 16 GB and now they game pretty much any game at high level graphics with little to complain about.