It's frontside as long as he's traveling towards the building. At first I thought he was coming towards us, but now I realized why everyone was saying frontside.
you guys are all fucking stupid. its a front side board slide. the front side part comes from how he aproaches the rail with his front to it. and its a board slide because he brings the nose of his board over the rail first. now if it were a backside boardslide he would have rode up to the rail with his back to it. and if it were a lipslide the tail of his board would have gone over the rail first.