This looks to be a pretty standard PC so plugging in a new graphics card (GPU) should be reasonably simple. We just need to know three things: your budget, the wattage of the power supply (PSU) inside, and the number of 6+2 pin cables which are called PCIE cables (note: not the 4+4 pin, which are called EPS cables). Assuming there's one 6+2 pin cable then a Radeon RX 6600 is your cheapest bet at around £200, with the RX 7600 (a newer model) costing £230 If there are no 6+2 pin cables then you will need to buy a new PSU.
Your grandson's GTX 1650 scores 3592 in 3DMark's Timespy, a popular gaming benchmark, whereas a RX 6600 scores 8144 and the RX 7600 scoring 10897 so both are a substantial boost. I would, however caution that the RX 6600 and RX 7600 may not be decent longer-term choice as they are rather limited cards by modern standards, at the bottom end of the new cards. The RTX 3060 scores 8703, again a big improvement over the GTX 1650 but not as good as the RX 7600 however it has more memory (12 GB vs 8 GB), and some models of the 3060 have 8 GB VRAM so you need to be careful.
Just to explain a little, the amount of memory on a graphics card (called VRAM) affects the quantity of textures (crudely, pictures of things) it can cache and modern games love textures. When a game runs out of VRAM it can stutter while it pages in the correct texture or display a low-resolution texture.
The more power, the more VRAM, the more you will pay.
I suggest you look at YouTube videos by Daniel Owen which I think you will find very informative. In particular he demonstrates the issues with VRAM.
Another channel worth checking because they test massive numbers of games is Hardware Unboxed.