Removing Co-Workers apple id from iCloud
My iPhone 5 broke the other day so I bought a 64 GB iPhone 5s (Software version 8.1) from a co-worker before he moved, but his apple id is still connected to iCloud. I live overseas so I can't contact him and I need the issue resolved as soon as possible. how do I remove his apple id from iCloud without the password so that I can use mine?
iPhone 5s, iOS 8.1