Illegal forward pass

Yesterday, While driving down the road, my wife calls me and asked me question about an NFL game.
Situation:
QB rolls out on a pass and crosses the line of scrimmage. He realizes that he is across the neutral zone an backs up into his back field in order to throw a forward pass down field. He was flagged for illegal forward pass. Is this a penalty in NFHS? I thought that any number of foward passes could be thrown as long as they originate behind the line of scrimmage? Not sure, any help out there?