Recommendations for 9-year old son.

My son is starting to get into PC gaming. I built him a PC several years ago purely because he liked the LED fans on mine and I used parts given to me for free by other redditors. But it's basically running on an AMD FX-8350 form 2012 with 16gb of DDR3. He plays Roblox mostly (with heavy supervisions...) but even that is sort of laggy at times. For Christmas this year I'd like to get him maybe into this decade so that he can at least play some lighter titles like Fortnite and similar social type games. His case is an ATX case and its newer, so it has decent features. He's got an EVGA 450 Bronze PSU, a 250gb SSD and I have extra storage devices if needed.

I'm thinking with RAM and GPU prices, an APU is the way to go here, but I'm open to CPU/RAM/Mobo/GPU suggestions. I would say $300 at most. Using fortnite type games as the use case, I'd like to get maybe 4-5 years of playing out of this at which point I can probably just hand down my gaming PC. The kid also has a Switch 2, so this isn't something where I'm trying to get the latest games to run. More like I just want the few games he does play with friends to run well enough and not be lagging because the hardware is 14 years out.

Thanks for your help.