Vast majority of time in my experience this is a footwork issue. It's not so much that's he's 100 MPH all the time but it's that he doesn't know when to dial it down to 95 MPH to catch a groundball with proper footwork.
Have him start thinking "quick - slow - quick" when it comes to fielding a ball. He needs to approach the ball in a quick manner then slow down to catch it and then get rid of it quickly. The slow down part is when he needs to dial it down to 95 MPH with proper footwork. This will help to smooth out how he looks.
I would have him progress from no groundball (ball laying on the ground) to a rolling ground ball from a short distance by hand to a hit ground ball from a close distance (hit it where it's a solid ground ball - not a screamer nor a slow roller) and then ground balls off a fungo / BP.
Have him start with a ball on the ground and have him approach it with the "quick - slow - quick" going on. This way he can focus 100% on the footwork. It's amazing how much you can accomplish when you take the ball away. One thing when he's doing the slow part make sure he shortens his stride down so he can do right foot, left foot plant field through and transition to the quick part to throw.
Then go to the next step of a ball rolled by someone. You can still focus on steps but now have advanced it with a moving object. Each step after this is about getting closer to game situation. He's got to get the footwork down pat to where it's muscle memory so he can focus on the moving object.
Hope this helps.