I’d suggest taking the mound out of the equation to help determine what may be causing it (mound or what he’s doing on it). Have him during flat ground catch add his leg lift and then eventually throw from his windup… Is the accuracy still an issue with each?
During regular catch is he sighting his target accurately keeping, eyes, " arm from shoulder to elbow" and finally "center of chest "all pointing at or going toward target… Does that change during windup on flat ground, Does it change during windup from mound? Etc.
Also, the times he’s thrown flat ground catch is probably exponentially greater than from a mound, and his muscle memory for hitting his target is that much more ingrained. Adding the mound and windup could change the natural things he does right from flat ground.
By the way, where is he missing, Arm side high? glove side low? That can help tell a lot. Does he block off with his glove side well while playing catch but let it fly open or back during pitching, because he is pitching and trying to over throw,etc.