By Charlie Wilson Las Vegas Raiders rookie Jackson Powers-Johnson accidentally flung the head coach’s son five yards backwards in his first training session as an NFL player. Powers-Johnson begun his training with his new teammates, making an immediate impression in blocking drills before one particular incident. During an offensive line blocking practice, Powers-Johnson did not hold back. Managing to collide with a bag-holding trainer who ended up sprawling five yards back, just about balancing on his feet. The victim of this over-enthusiasm was Deandre Pierce, the son of head coach Antonio P…