I know you said you don’t want Linux suggestions, but I’ll be that person and suggest a Linux distro anyway, in case you ever change your mind.
Ubermix is a Ubuntu-based distro (meaning there’s a ton of documentation and support) built with education in mind. They also have a nifty feature to easily recover your system from boot if you do manage to bork the system.
If your goal is to help your nephew get good at computer, fixing various broken systems over the years taught me probably most of what I know about how computers work. I wasn’t much older than your nephew the first time I made the family PC unbootable (I was trying to “downgrade” our “Vista-capable” PC to Windows XP, apologies if this anecdote makes anyone feel old). It took me a couple of days to get it running again after searching through YouTube and forum posts, but that rushing sense of accomplishment when it finally booted up to the bucolic green hills of the Bliss wallpaper got me hooked on troubleshooting and tinkering with technology for life.
Either way, best of luck to you and your nephew, it’s really cool that he has someone supporting him in his interests! You’re good people.