Still waiting to get adopted by you. I want a computer JUST like that for Christmas too.
As far as monitors go, you're looking at:
Size (27" is the sweet spot if you're going for a traditional aspect ratio, of you can just go crazy on the ultrawide)
Resolution (2k vs 4k)
GSync for that juicy Nvidia GPU
Panel type (IPS vs VA vs TN, etc)
Refresh rate (120 Hz+)
HDR options
Good monitors cost a lot but you want to do your son's beast of PC justice.
If you think he will be doing 4K on it, you probably do not need a crazy high refresh rate because even the 3080 can't push 200 frames per second at 4K.
If he will be doing 1440 (2K), you probably want to get a high refresh panel with Gsync for that buttery smooth image.
Panel types are probably less important, but IPS is probably still the best as far as I know. IPS produces the most vivid colors, and has good viewing angles. I honestly just don't think your son will care if you get a TN panel, though.