This is complete, but over budget( though i put everything in including the case, cooler , ssd etc). I'm assuming as your son wants the 3500x case that he wants to see in and have some rgb lights to look at also
Therefore went with the b650e-f mobo, as can be controled by Icue
..also has pcie5 gpu slot/m.2 slot for upgradeability in years to come(you never know)
7800x3d can be air cooled but put the tital link 240 aio cooler in...the titan coolers are actually good from review i saw. Again, the fans are the same that come with the 3500X case and are link fans, just as case ones are, so nice and easy to put together
as corsair, pretty much had to go with the corsair rgb ram, so expo 32gb 6000c30
2tb ssd as games are quite large
it was on offer but can easily be changed but rm750x shift psu..makes easy cable plugging in...though truthfully, even not needed for this I'd go something bit more beefy if your son upgrades gpu in few years, but then again the 4070super should be good for fair few years, esp at 1440p and with dlss
4070super looses 4gb vram, but is faster than the 7800xt, better at raytracing by far and dlss is better than fsr(this is just an alternative to 7800xt which is cheaper etc and still a good card)...guess you get star wars outlaws if you into that game for free on top which helps with the additional cost compared to the amd
there are cheaper cases etc to bring costs down, and could use air cooling also on 7800x3d(i do), but this ties it all in together if your son is looking for a complete look...this way icue controls everything
My basket at OcUK:
Total: £1,720.87 (includes delivery: £0.00)
you could pay £20 more and get the asus gpu too if you really want to tie it all in together, but don't think it makes much of a difference