You will need to reset the BIOS by disconnecting the CMOS battery.
Download the service manual and get your small screw drivers ready.
[link|https://download.lenovo.com/consumer/mobiles_pub/legion_y540-17irh_pg0_hmm_201904.pdf|Lenovo Legion Y540-17IRH Service Manual|new_window=true]
Once the case is open disconnect the main battery as shown on page 37. What you need to do next is hard reset the BIOS by disconnecting the CMOS battery as shown on page 38 of the service manual under "Remove the coin-cell battery". Press the laptop's power button to dischage any latent power. Reconnect the "coin" battery and the main battery, close the case and power-up. You should be able to get into the BIOS screen by the normal method.
You will probably have to reinstall Windows by down loading the installer onto a USB flash drive. Microsoft can help here or my choice use Rufus v3.22 to make the bootable drive. The Window's installer has an option the save data files. It may or may not be important to your son.
Please don't hesitate to ask if you have any questions