To better understand the situation, could you please open Settings > Accounts > Your info while signed in to the Windows profile containing your business documents and confirm whether your son's Microsoft email address appears there? Also, do you see the option Sign in with a local account instead? This is important because your son's Microsoft account may have become linked to your existing Windows user profile, meaning you might be able to disconnect it without creating another Windows account or losing access to your files.
Additionally, could you confirm whether your son's account appears under Settings > Accounts > Other users, or only under Your info and Email & accounts? When you previously removed an account, which one did you delete: the newly created administrator account or the original profile containing your business documents? Please do not delete any additional Windows user profiles until identify where your files are stored and ensure they are backed up.