I can't log into my kid's account

I have my own account, the one I'm typing this on.. And my son has his own account that uses an email address linked to an old account of mine. His own account and my old account share an email address, and whenever I try to log into his account, it just logs me into my old account. I can log into his account through the app, but can't do anything. His account is connected to Steam, and when it did let me log in, it did show his one game, star wars battlefront 2.. But then it stopped working and only shows my old account I don't use. I don't know how this works.. I've been trying to figure it out for over an hour.

So, I have this account.
I have an old account through a gmail address. I used tat gmail address for my kid's account. When I try to log into my kid's account, it only shows me my old account. 

I want combine my old account and my current account, and have my son's account linked, but I can't figure out how to do it.