AMD CPU supply is very poor ATM,so prices have shot up and availability is poor. For example a Core i5 10600KF now costs close to what a Ryzen 5 3600/3600X costs and is quicker.
However,as your son is streaming,I wouldn't get a 6 core CPU now - even the Ryzen 5 5600X which is quicker than the Ryzen 7 3700X,starts to struggle in a number of games when you stream.
So I would suggest getting a Ryzen 7 3700X/3800X as long as it is under £300. Another alternative is the Core i7 10700F if its a similar price.
WRT to RAM,3200C16 or 3800C18 would do the job. If you can get the Crucial Ballistix 32GB 3600C16 or 3200C16 kit would be ideal due to the use of Micron E-die but they are hard to get hold off now.
With AMD,an MSI B550 PRO-A or Mag Tomahawk would be good options if you want to use an ATX motherboard.