Household game sharing

He would either need to continue using your account and you create a new one for yourself or you'll need to create a new account for your son and he would have to start over.

I'm sorry but assets including achievements are not transferable between accounts

It depends, because Epic Games doesn't allow you to play the same Fortnite account at the same time. It will remove the other Fortnite Game session from Gameplay. If you're trying to play Fortnite with your own account, you'd need to unlink the Xbox account from, in your current situation, your son, and then you'd need to make a new Xbox Account for your Son, and link the new Xbox Account to his Epic Games Account. You will, however not be able to see your son's Fortnite Game progress, since you've transferred it to a new Xbox account. I'd recommend you to follow up with Xbox Support and Epic Games.

I should be able to network share the games.... or that is how I read in network sharing....correct?

You do not.

With the home console/game sharing you can make one Xbox your home console and when you login on another all your games and benefits are available on that console.

That does leave the third console out of the mix though.

how to designate a home console