Considering that he technically dies if someone saw him without the mask, he takes it off at every possible opportunity and puts it back on as late as possible.
Teleporting in front of the demon king castle and only putting the mask back on when he is nearly in front of the gate? Wonder that not every grunt and his mother knows that he is a human.
Edit: Plottwist: He is the only one thinking that this is a secret and nobody really cares if he is a human or not.