If you watch the video and listen closely, there are certain parts of the song where it's obvious that he's singing live. I understand how you might not think so, because in the beginning I thought it was lip-synced too. However, later on in the video you can really hear his voice come out and it's slightly different from the album version. There was obviously a backing track -- it's just not clear if he was overpowered by the backup vocals or if he only sang certain parts of it live. Either way, he is EXTREMELY talented vocally and dance wise and there is no denying that.