Hi Emma, thanks for getting in touch, I'm sure we can help provide advice.
First thing I'd say is that this is not the best time to buy a gaming PC. Certain chips and parts have become very low in supply, so prices are ridiculously high at the moment. So bear that in mind.
Secondly, do you know what type of games your son plays?
Thirdly, do you have a monitor? If not, that would be needed too, and the resolution, be it 1080p, 1440p or 4k would impact what type of PC to go for.
Finally, there are sites like Overclockers that sell ready-made gaming PCs and I encourage you to check them out.
If you reply to the questions above we'll be able to see what's best.