XCOM 2

The best game ever made is Baldur's Gate 3, so that's what I'd give if he hasn't played it. IDC what kind of games he usually plays, that's what I'd give.

For PC I would go for the avan-garde stuff

e.g. Cold line

Most popular indie titles are already on console as studios will port stuff to increase the sales.

I would go to look for stuff that can't be sold. Dream like avan-garde surrealism or under 2h experiences on itch.io and newgrounds.com

also RTS require mouse so it could be a fun stuff to get into.

SuperHot

The DOOMs

I thought the old homeworld games were great. Kind of RTS in space ships. They're old now but still good.

Game bundles like from Fanatical are a good start. You can't return them if you have issues, but they're great if you haven't played any of the offerings given.

A similar thing would be Xbox Game Pass, but that very much locks you into their ecosystem, and they're clearly done investing into worthwhile games.

Tormented Souls was really good

The Sonic Racing games are the best counterpart to Mario Kart available on PC. Sonic Racing Transformed is the best of the 3 games (and bundles the very first game) but sometimes gets confused when switching up controllers, so you must delete the config file, so maybe because of that not the best introduction to PC. Team Sonic Racing is a bit less good but that input bug is gone.

Total war: Warhammer 3? Baldurs Gate 3? Maybe those are available on console as well..

Might consider getting something that only plays well on keyboard/mouse. Factorio comes up off the top of my head. Shooters also generally translate poorly to controllers, so perhaps that?