Help with Activation lock

We don’t know the iCloud account we originally set up the phone with so this won’t work. It says the phone is linked to an iCloud account and then wants us to use that to sign in and we don’t know it. It was my sons phone from 3 years ago and he’s had so many different #’s and accounts.

I don’t have proof of purchase - it was in 2018 - I don’t have any way to reset it - this is so frustrating- I used the phone for 2 1/2 years and it didn’t do this when my son gave it to me and erased it. It’s not a stolen or lost phone either. 🥲