Be sure the SHOOTING MENU is set for both
"Servo AF" - ON
"AF Mode" - CONTINUOUS
The camera should do well as far as exposure in daylight - as the previous poster said, at night the lens most likely will not be fast enough when zoomed (should be OK when not zoomed and wide open for close shots - f2.8).
Shooting indoors is not that difficult in a professional arena lit to be a huge TV studio. Shooting in most high school gyms may prove to be futile (all you can do is try).
Shooting outdoors should not be a problem in daylight. If your son plays football games during the day, the camera won't be the problem. The right settings will allow the camera to stay focused on a moving subject.
If the games are under lights, all that can be done (most likely) is take shots from the sidelines NOT zooming in much so the wider apertures stay available.
Human eyes will perceive 10 times as much light as being just twice as bright - Cameras will perceive 10 times as much light as being 10 times as bright. So while a high school gym or football field may appear to be well lit, that is your eyes and your brain compensating. Cameras will see (for example) light that is 4 times as bright as being just that - 4 times brighter - a huge difference for exposures. But your eyes are not likely to perceive a noticeable difference at all.
Don't give up - get closer to the action and use less zoom.