Get back my Windows 10 Product Key

My son's machine had Windows 8/8.1 Home and updated to Windows 10. Someone told him to use a random Windows 10 Pro key and he would get Pro for free.

It didn't work obviously and now he see's a "Activate Windows" watermark on the screen and has an un-activated generic pro key tied to this computer.

He doesn't even need pro and I was wondering if it's possible to get back to the home key that came with the laptop. The whole activation/product key thing is very confusing now that it's in the BIOS. They didn't ship the computer with any key that I can find.

Is it possible to find the home key that came with the laptop? Would it be on the USB backup that we made?

Any help or pointing me in the right direction would be great.