Unable to Connect Accounts

Hello community, I've been running in circles and could really use your help. Here's the scenario. I have an XBOX account (Campbell126) and I have one Game Pass Core and one Game Pass for Console subscription. My XBOX account (Campbell126) is connected to my Epic Games account (Fortnite). I have logged into my epic games account and verified it is in fact connected to my Xbox account.

Now, my son had his birthday and got a bunch of Fortnite V-Bucks. So last night I created him his own XBOX account and added him to the family on the console. I thought adding him to the family was the right answer but not entirely sure. I then created him his own Epic Games Account and attempted to connect it to his newly created XBOX account. However, when I attempt to do so, I receive the following message:

'Unable to Connect Accounts'. Your Xbox network account was connected with another Epic Games account. Please log in using a different Xbox network account, or switch to another Epic Games account'.

As I mentioned, I just created this Xbox account for my son, so I don't understand how it could possibly be connected to another Epic Games account. Is the issue maybe that my sons newly created Xbox account considers this machine to be his 'home'. And that MY Xbox account (Campbell126) considers it as 'home' as well?

My goal is this. I want to be able to log in with Campbell126 and play Fortnite with my own progress, skins, etc. And when I'm not logged in, I want my son to be able to log in using his own Xbox account and play Fortnite with HIS own progress, skins, etc.

Is this achievable? Seems like a basic thing to be able to do. Thank you in advance for your help. This is driving me nuts.

Update: Additional Details

To be clear, my son can log into his XBOX account and play Fortnite. The issue is that I cannot redeem his V-Bucks via the site because of the error message I receive above and the error message I receive below. Additional steps I've taken. I use my laptop (not where the game is played) and go to fortnite.com/vbuckscard and log in using my sons new Epic Games account. Once logged in, I select 'Get Started' and enter the V-Bucks pin. It then asks me 'where do you want to use your V-Bucks. I cannot select the XBOX option because it says, 'This device does not appear to be linked to this account. Please check that you entered the correct account.' So I assume the issue here is that I'm attempting to redeem V-Bucks via my laptop and not the XBOX itself. I then use Edge on the XBOX itself to navigate fortnite.com/vbuckscard and I get the same exact results. 'This device does not appear to be linked to this account. Please check that you entered the correct amount.'

:)