The biggest handicap you're going to have to deal with when using that lens for sports is the relative narrow maximum aperture. At the longest focal length, which you're going to be using most often, your lens will be limited to f/6.3. Even if you are in bright daylight, which will allow for proper exposure at the shutter speeds you'll need to stop the action, your camera will be operating on the edge of its ability to autofocus.

Cameras such as your D5100 meter and focus with the lens wide open and only stop down to the selected aperture the instant before the picture is taken. One of the reasons they do so is to allow light from the edges of the front of the lens to fall upon the AF sensor which works by comparing the light from opposite sides of the lens. The wider the baseline from one side of the lens to the other the more accurate and faster the AF can operate. When a lens with a maximum aperture of f/6.3 is used your camera will not focus as well as it would with a lens with a wider aperture such as f/4 (which is over twice as wide as f/6.3) or even f/2.8 (over four times as wide).

You can help your cause somewhat by using either a half press of the shutter button or, better yet, by using back button AF to pre-focus your lens on your son just before each pitch so the lens is already very close to where it will need to be when you shoot. Set the camera to AF-C focus mode so the camera will continuously update the focus distance from the time you initiate focus until you press the shutter button down to take the photo. For more about back button AF and its advantages, please see What does the AE/AF lock button do that half-pressing the shutter doesn't?

You'll probably want to use a shutter speed of at least 1/500 second (1/1000 is even better for baseball) and then set the ISO you need (either by trial and error using the camera's meter or by selecting Auto ISO while using S exposure mode) to get proper exposure for the amount of light you have. At f/6.3 you'll be pretty much limited to daylight to get good image quality within those constraints.