In the US for copyright to transfer he'd need to be an employee and writing part of his work duties, in which case it is work for hire, or if he is a contractor they'd need a signed contract saying the contractor was performing this as work for hire, or a contract explicitly transferring ownership. They did pay him for it so Mojang had an implied license to use it but he retains ownership.

When Mojang sold Minecraft to Microsoft, they did not own the copyright and did not have a contract saying they have the right to transfer their implied license, therefore Microsoft actually is violating copyright here.

Source: I went through exactly this same legal scenario and won.