But perhaps things I know from then could help:
1. Can he use a "faster" film (in my day, it was "Tri-X) for dark situations and 2. I don't know if it is still possible now but back then you could "push" a film to be faster by developing it slightly more when developing the negative. 3. "Grainy" can work in your favor but print it on a (what back then was called) number 6 type paper (very black and white, really no grey tones). I did a photo around 1967 of a crowd at my high school, enlarged it like crazy ( = lotsa grain) and printed it on #6 -- and -- guess what? -- the Class of 1968 chose it as the photo for reunions. I was pleased as punch. Good luck to your grandson.

Also, he doesn't want to print the photos. He wants to have them used in the school's newspaper. The paper in school newspapers isn't high grade....

Then I wonder.... is it even on paper? I'll ask him.

ISO looks to be "6400, extended mode to 50–12800, HI2 mode 25600. " high iso gets more "grain"

It's a full size sensor, likely better than a smaller one for collecting light.

There should be lots of fast lenses for the DSLR F-mount, a very large marketplace, even used ones.

Fast AND long focal length drive up cost. Get a close seat.

And that's about it. Shutter speed will be obvious in the results.

No generic info to offer right now. That's a fine camera. Being light-limited, mode should not a big deal. He'll probably be shooting wide open and at the fastest shutter speed that light allows no matter which mode he's in.

I also have a 100 mm F2 and I used to shoot HS wrestling for the paper. The ISO was a lot slower back then.

It'll be a while (maybe birthday) before he gets a different lens. He has a Sigma 100 -500 lens which is useless for this purpose. And two small lenses that I don't know what they are. I know about he Sigma because I gave it to him.

Moving figures in low light. He maybe using the Auto features on his camera, in which case the camera is pushing the film speed, or ISO way up to 1600 or more, this makes the images more grainy. It also has to have a shutter speed fast enough to catch the action. Longer shutter speed lets more light in, but is harder to keep the camera still. He should play around with the manual settings and see where he can get a good image. There are many good YouTube videos to explain these things if he needs.

(I understand the camera is capable of 1/1600 or 1/2000)
if he is setting the shutter speed and letting the camera set the film speed (ISO) he might try to slow down the shutter. He might still be able to catch the action at 1/200 or 1/300, and the slower film speed could make it less grainy.
He also should have the aperture as wide as he can get it. 3.5 or 4. This lets more light in and helps get a slower film speed.