Get Epic Games Launcher for your PC, there's new free games every week. 

 Interesting idea. I'll try that. The only downside of that is that I'm on Linux so it's likely that he won't be able to play with me 

 Use heroic launcher.  

 I think its easier to get kids hooked on console games first. Like my six year old loves Minecraft, but only ever played on console, and only offline. I'll be building a new pc for myself this year and giving him my old one and I think it will go well

If your kid is anything like mine, dudes prob got a strong preference for co-op games, not competitive. So if not Minecraft maybe try burnout Paradise 

 How is he bored on a PC? There are nigh infinite free games. Back when I was a kid, SSF2 was peak PC gaming. Or coolmathgames.

In all seriousness, I'm not sure what to say. You don't always need new games. Games like Minecraft provide limitless playtime that can be expanded by mods.

Also maybe introduce your kid to various other ways to pass time. Like you don't always have to be playing video games, you know? 

 Minecraft and Roblox have innumerable games-within-games if you want to go down that path.

My kids play whatever their friends are playing, usually some ad-supported iOS app... :-/