Cant Login to Account - Know Username but not email address

I created an account for my kid on PS5 and want to link his APEX game to his Nintendo Switch 2. I know his EA username, but I cannot login to his EA account with just his username. I use the email address that I believe works but it says the email address is not recognized. If I do a forget my password, I can use his Username to change the password and get an email with a verification code to My adult EA Games email address. This is not changing the password to MY ADULTA EA account as I can still log in with the old password, but I dont understand how to log in to his account. Ultimately what I am trying to do is link his EA account to his Nintendo Switch 2, so he can play APEX on his account with all his skins.