Q: Locked out of iPhone 5s after battery problem.

To start off, this is my Dad’s account that I am writing under and looking for support. I was fixing my Dad's iPhone 5s because the phone wasn't charging properly, so I put it into DFU mode to see if that would work. It seemed to be working fine and the phone got restored, but now I'm locked out of the phone because I don't know/remember the iCloud email. He has a different Apple ID that I created for him, but the phone seems to be locked with his iCloud email. How would we go about finding the right iCloud email associated with his phone? I already tried https://iforgot.apple.com/ and it's only showing his Apple ID. As a side note, under his Apple ID it shows the phone is one of the devices he manages. Any help is appreciated, thanks.