how to get rid of "owner" on bootup

I am gifting this computer to a son. It works fine with Windows 10. I added his name as administrator and deleted my account. Everything still fine except when the computer boots, it shows an Owner account and his account. When I click on his account and type in pin, computer works fine. I just want to get rid of the Owner account as the computer tries to boot from that account first. I don't see an Owner account when I look at users, only his account--as it should be. Yet the computer thinks there is an Owner account also. How do I delete it if I can't find it?