Cannot buy Minecraft Java for son's new account - says Already Owned

I'm trying to set up Minecraft for my youngest on a laptop which already has it installed and has been used by other people.

I created a new MS account for him which is a family one.

When I start the Minecraft launcher and log in with his account, we can play Bedrock but for Java Edition it says 'BUY NOW'. If I try and buy it, I select which one I want (Normal/deluxe edition), then there's a Windows security thing which wants the PIN for the laptop (not his account), then a pop-up says 'Already Owned' and that's the end of it.

How do I get him playing Java edition? I don't mind buying it again if we have to. Nobody else uses the laptop now so I can delete other accounts or whatever.

:)