Can't remember original Epic Games account information, how do I redeem V-Bucks?

I got my Xbox One a few years ago (I believe in 2018) and I don't remember setting up an Epic Games account when we downloaded Fortnite, but I seem to need it when I want to redeem V-Bucks. Every time my son get's a V-Bucks card it's a very long and ridiculous process that results in calling customer support to redeem them.

He plays on my Xbox account which uses my email address. I just tried to redeem the V-Bucks using that email address but it said an account doesn't exist. I set one up, but that won't do any good since my son has a lot of skins and achievements on Fortnite already.

How do I redeem V-Bucks in the future so that they upload straight to our existing Fortnite game on my Xbox account? Is there a way to link my new Epic Games account without losing all of his skins and progress? Or is there a way to find out what Epic Games account he's currently using?

:)