Is it glove side, lefty hitter, or both? I'd guess that pitches in on a righty are where he's having trouble. Pitches that are inside, particularly up and in need to be caught. They can hit the umpire or give up bases if there are runners. Something he just needs to work on. Usually if you are tracking a ball you can get your mitt in position without seeing the ball into the mitt. He might be rising up and cutting off his line of sight... if so try working on staying down and just reaching up for the ball.