In case anyone besides WR's son has Windows troubles.
Your best insurance is a disk image backup using Macrium Reflect free or paid. That's the only thing that always works -- everything else is try & see what happens. You may get lucky -- you may not.
For boot problems there's EasyBCD, which will also work for you if you boot to a USB stick, e.g. running WinPE. The official CLI tools are BCDBoot to add the BCD boot files, & BCDEdit to change them -- both are included in Windows. Use their native help if possible, /? . Paragon, Macrium, & I believe some AOMEI products, the paid advanced or pro versions, can create a bootable USB stick with boot repair tools.
If you cannot start Win10 successfully, after trying several times you *should* be presented with a boot menu with several repair options. The default boot menu is always there, but it's usually not shown when there's only one OS installed -- you can change that using EasyBCD. When the regular boot menu is displayed you can hit the F8 key to get to those repair options.
And/or you can try using DISM, which is the almost sort of a Swiss army knife for working with .wim files. That file format was developed by Microsoft to hold most of the files used to boot & run Windows 10, regular or WinPE. When you install Win10 the Windows files are all in a .wim file in the ISO, and a copy for Win10 repair is stored on the default Recovery partition. You can also make a .wim file copy of a complete Windows installation, then use that to deploy Win10 on however many devices you wish. DISM is included in Windows, and has a few options to repair or modify a Win10 installation.
support.microsoft[.]com/en-us/help/929833/use-the-system-file-checker-tool-to-repair-missing-or-corrupted-system
docs.microsoft[.]com/en-us/windows-hardware/manufacture/desktop/repair-a-windows-image
docs.microsoft[.]com/en-us/windows-hardware/manufacture/desktop/deployment-image-servicing-and-management--dism--command-line-options