Yes HP aren’t the only ones, I have seen it on others too notably Gigabyte for relevancy.

Most of the symptoms point to cmos battery and normally it’s an easy replacement. Not sure if you feel up to the task but for the B460M DS3H AC the battery is located under the first pcie slot, probably where the GPU is, so you would likely have to remove the GPU to access it if there is not enough wiggle room, then put it back together.

There is a diagram here https://download.gigabyte.com/FileList/Manual/mb_manual_b460m-ds3h_e_1003.pdf

I’ll say again, complain to the tech. If he really did re-install windows from scratch he shouldn’t have installed in legacy mode. There is zero reason to not use uefi on modern hardware.

Prime example for your sons use case, if he ever wants to play the game Valorant he will need to enable secure boot which cannot be done in legacy csm mode. You don’t have to re-install windows and you can convert it to uefi, but this would not have been an issue if the tech did the job right the first time.

Edit: just for clarity’s sake heres a link to a guide on how to convert a legacy windows install to uefi, you don’t have to follow it if you’re uncomfortable with any risk. This would technically solve the current issue as well as you can see in the guide it converts windows so you don’t have to keep changing the legacy setting

https://answers.microsoft.com/en-us/windows/forum/all/convert-an-existing-windows-10-installation-from/aa8c2de3-460b-4a8c-b30b-641405f800d7