Not your son's fault. Apple should have just made this work. Try some of the tips posted here until one of them works. This is recoverable, you just need to find the path that will work for you.
khank
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Another option that I saw mentioned elsewhere: Use the Option key during startup for Startup Manager. (Let's you connect to wifi and choose a disk to boot from; not working for me yet, but just keep trying)
Also, for those who can get into Safe Mode and are panicking because they don't have a backup of their data. You can plug in a USB drive once you're in safe mode, and find your old data in /Data/Previous System/... For peace of mind, you can back up your files, and then do a clean install. I may go that route myself soon.
The comment at the bottom of the page appears to be the official recommendation from Apple.
https://support.apple.com/en-au/HT207019
Namely, "If the issue persists, press and hold the power button until your Mac turns off. Then unplug all accessories from your Mac, including printers, drives, USB hubs and other non-essential devices. There could be an issue with one or more of those devices or their cables. Then turn your Mac back on."