Yep, the software geeks site is the place to go, Win10 Pro has a few extra features over the home version so no disadvantages, you’ll get a download link and a license key emailed to you when you purchase, I’ve bought half a dozen Win10 licenses and never had an issue.
I’d just create an install USB using the Win10 media creation tool and wipe the install drive during the installation process, that should clean everything up for you.
As for minecraft, the Win10 version uses the Bedrock engine that is used for the mobile and Xbox versions of minecraft, it means that he’ll be able to cross play with mates who have it on Xbox. One the other hand the Java version has far far more mods and maps available for it but you’ll need to help moderate getting and installing them as a lot of them are hosted on dodgy sites, add into that the hassle of keeping java up to date, the fact that the mods are all version specific so you need the right version of minecraft installed for the particular mod you are playing and it can be a bit of a ball ache staying on top of it.
Unless he’s desperate for Java specific mods I’d say the Win10 version would be less hassle for you, mods are appearing for it at a fast rate, the engine is getting more active development and you just download them from a built in mod ‘store’ (although this does mean that some of them are paid for rather Java version where they are all free.)
If you fancy the Java version, look for a launcher called MultiMC which is like an organiser for minecraft versions, you can create multiple instances of minecraft of differing version ps and add mods to each one separately (my lad has about 30 instances of minecraft setup on it!!)
Sorry for the huge post but I’ve gone through all this over the last couple of years as my eldest (11yo) is obsessed… 🙂