OK. Right now you'd need an AMD RX 7900 XTX + 5800X3D to play God Of War 4k/120fps natively using Ultra settings ($1500-$2000 PC build). An RTX 3070ti can do 4K/50fps native and reach 4k60fps using DLSS, but definitely ain't doing no 4k/120fps. To put this into context, PS5 runs God of War @4k/60 CBR (Checkerboard rendering) That's definitely not native. The PS5 also runs God of War Ragnarok @ 4k/30fps (Native) and 4k/60fps with dynamic scaling (1440p-2016p) This is all according to Digital Foundry (Videos are bookmarked).
The GPU equivalent to what's in a PS5 is an RX 6600XT with extra vram (16gb instead of 8gb). From what I'm hearing regarding the PS5 Pro, its GPU should be the equivalent of an RX 7700XT (This is what is impressing people), but with the same CPU from the regular PS5. That GPU/CPU combo ain't doing 4k/120fps native for AAA games. The old CPU will be a bottleneck.
Have you tried hooking up your son's PC to the same 4k TV that you use to play PS5 games? I promise you, you'll be shocked how much better your son's 3070ti rig compares to your PS5 playing the same games. PS5 games aren't the same as a PC game using Ultra settings. Here is comparison of God of War on PS5 vs PC using Ultra settings. There's about a 10% performance difference between a 3070ti and a 7700XT. If your son's PC has a much better CPU, then his system should give the PS5 Pro a run for its money.