Hi, you will need a copy of Windows 10 (Home edition will be fine) You can probably still buy the physical disc from a store, but you can also download the file from Microsoft store
in Windows or their website and put it on a USB memory stick from another computer. If you put the memory stick in the new tower and boot it up it should find the media and begin the install automatically. Just follow the intructions on screen.
I understand you can run Windows 10 unactivated for as long as you need, although I havent done this personally. Apparently the only penalty is you will get notifications telling you to activate it.
Its not cheap 139USD, but you do need it, unless you want to use Linux, another OS which is free, but that isnt suited for gaming without some wrangling. You might find an OEM version of Windows a bit cheaper, but they arent transferable if you ever upgrade the motherboard you'd technically need to buy another copy. With a full copy I believe its unlimited or at least may as well be.
There are also grey market keys you may find for very cheap around the net. I don't think they are worth the risk personally, as they may be revoked without warning if they are reported stolen or otherwise illegally obtained.
To the other part of your question. I've studied myself using MS Teams, Office 365, and another class used Moodle, so I assume whatver your sons school is using will be accessable from his PC quite easily, or everyone would have problems. The school should provide intruction. My school had access to MS Office 365 through the internet browser via a student account.
You can also use something like OpenOffice,
which is free software compatible with Microsoft Office and upload files from there after creating them locally on your machine.