A Microsoft file hosting and synchronization service.
To stop the files syncing from the family PC to the new laptop, your son needs to stop OneDrive syncing on his account on the family PC or on the new laptop.
When setting up the new laptop, there are many syncing settings that will appear after he enters his account details, he must ensure he does not choose to sync files from the PC, then as soon as he gets into Windows on the new laptop, either open OneDrive settings an unlink the new laptop from OneDrive, or uninstall the OneDrive app to stop the files syncing from the family PC.