Hitting .328 is not a slump!!! 28 at bats is not enough if a sample size to judge anyways. If he was 14 for 28 that should taken with a grain of salt, too... Stop fretting about it. It is 13U, it is not a big deal. Do stuff that is fun. HR Derby is a great idea. Have him throw a left handed bullpen. Seriously. Just let him be a kid and keep it fun. Continue doing the lessons etc but focus on enjoying the game.