Hi and welcome to the group. You have an excellent camera that should provide superb shots of your son on the football field. Matthew has provided some helpful tips. I am not familiar with your camera but I shoot soccer in the sports mode with uses a high shutter speed. This shot below was made a 1/1000 second but I was right on the sideline and only shooting at 78mm focal length (not using telephoto).
It would help if you could post a link to some of your shots that are blurry or out of focus. Also by examining the EXIF data (ISO, shutter speed, f/stop) we can see how each shot was made. Are you using the image stabilizetion mode and is it turned on? This mode will not help if you are panning with the action. As for focus, you may not be allowing the camera to auto focus and are quickly depressing the shutter button in one swift motion as opposed to the half-way down to focus and set exposure. It may also help if you can get closer to the action. By using a telephoto, any camera motion will be greatly amplified. I look forward to seeing some of your photos.