I need help unlocking my iPod touch

My iPod touch has an activation lock on it and my id and password aren't unlocking it. I don't see the device under my account devices. I originally purchased the device in 2016 for my son. It's linked to his account and I only know this because I looked back at his emails and saw that. He misplaced the thing two weeks after Christmas and two years later we found it in a winter jacket. He didn't need it anymore so I thought I'd sell it. I erased and reset the thing to factory settings and left it alone for a while. The other day I was ready to sell it and tried getting into it and that is when I discovered the lock.