It's easy to tell when they mime, you just have to pay attention to their microphone.
Shayne started off live, in the first verse he pulled the mic. away from his mouth a mere moment before he stopped singing and you could hear the change in volume.
I think he was miming to a pre-recorded chorus, or a very loud backing track swamped over his voice. At one point he took the mic. away from his mouth and you could still hear him singing (final chorus), that tells us that it was either a pre-recording, a loud backing track, or the backup singers.
Choruses are almost always mimed in pop songs.