@SophieBxnks The point of getting the higher-end components (motherboard, power supply, CPU and CPU cooler) is that the computer would only need one upgrade at a time. For example, this build would support any graphics card currently on the market, even the £1,200 Nvidia 2080 ti, not that your son would have any use for it. But in two or three years, he could swap out his current card for whatever is new and shiny without worrying about anything else.
Installing a GPU is pretty simple, even if it seems intimidating for someone who's never done it. Here's a video:
https://www.youtube.com/watch?v=mG0GZpr9w_A&feature=youtu.be
The other option would be to take the computer to a shop and ask for help. If you bought the card there and asked nicely, they'd probably install it for free or for some low service charge.
As for the card itself, an Nvidia 2060 is already overkill for Sims 4. (The cards that are £100 cheaper are already more than enough for the game.) There's nothing wrong with that, but spending an extra £200 or so on a more powerful card isn't going to make any difference to the game, now or ever. So there's really no point in getting a better card until your son has some use for it and decides how powerful that new card would need to be.
As for the build overall, I can keep adding better and better parts if you want: a solid state drive from a more respected brand (but the same speed), a second SSD instead of a mechanical drive for extra storage, or a 1 TB primary SSD instead of 512 GB; a motherboard with more slots that your son will probably ever use, but the same overall technology as the one I initially picked; lots of completely unnecessary RGB lighting.
But without knowing what your goal is, aside from budget, it's hard for me to know where and how to apply the upgrades. If you really don't know, I can just pick the best parts up to £1,500 or close, but I'd rather choose upgrades that will be worth the money to you.
And just to reiterate, hardware in this class is not in any danger of faltering while running Sims 4. As long as the hardware itself isn't somehow defective—always a risk with any kind of computer, but that's what warranties are for—your son should never have any issues playing the game. It might still crash here and there, because all games crash occasionally (mine crashed just yesterday, and I laughed), but it won't be the computer's fault.