Can't upgrade child account to Teen

I started the process and couldn't finish entirely as he wasn't present (needed his phone to sign into a Google to get his email).

Now when we got to his account either via the Family Center logging in as me, or via the Meta account page, it congratulates him on his birthday then gives him the details.

From there it gets to a page to sign in with his email address pre-populated.

Before I can enter a password I get:

This email is already associated with a Meta account, so it can't be used to create a new one. To continue, log into your existing Meta account. If you used Facebook or Instagram to create your account, you'll need to log in with Facebook or Instagram too.

This happens when I go to https://accountscenter.meta.com/ and log in as him with the password I set. I can change said password, and have said password up to date.

There's literally zero support that I can find on this, the closest I can find 

For clarity, this is the process I'm going through

After this comes the screen at the top.