Thanks and good luck tonight (both you and your son!)
Good field lighting definitely helps. Try setting your camera to Av mode with the lens wide open (F4), set the ISO to 6400, and see what it gives you for shutter speed. To me, 1/500 is about the slowest usable for action sports like football and soccer but higher is much better. If the camera selected shutter speed is slower than 1/500 then bump your ISO up. OR if the lighting is unusually good for a high school field and ISO 6,400 results in 1/1000 or better then decrease ISO but that won't happen unless the lighting is really good.
Also experiment a little tonight shooting with the camera set to manual aperture and shutter control with ISO set to auto. With the aperture wide open @ F4, try a few shots at 1/500, 1/640, 1/800, and 1/1,000 (if there is enough light to get a standard exposure within your camera's ISO range) and decide once you process them what settings give you the best overall results of shutter speed and ISO noise. In general, significant noise can be reasonably addressed in post but motion blur cannot and so when necessary bias towards higher shutter speed and higher ISO when necessary.
The second photo in my games shots above was shot at 1/800 ISO 16,000 and the third was at 1/640 ISO 8,000. Last year the team played an afternoon game at Memorial Stadium in Champaign and it was so nice to be able to shoot at high shutter speed and native ISO but that doesn't happen often with HS football. My daughter also really enjoyed that game because it was her first time to see her HS team play in Memorial stadium and I got a sidelines pass for her also.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
