I posted this comment not realizing that the OP was on the pro-chef forum. But having gone through the trouble already I figured I might as well post it here. The problem is with a cook who can't make proper cubes or battonets, although his fine cuts are perfect.
I've done some original work on motor control issues. Small cuts are easy to judge in terms of thickness and uniformity, its the large cuts will present dimensional problems. A person needs to be able to clearly visualize in their head shape and size in order for there to be any kind of consistency.
Google "athletics and mental imaging" for the details. Then you just have to get him to put a few minutes into studying it.
Aside from that you can check out his basic mechanics as suggested.
It's possible he may also have visual perception errors, but the mental imaging can compensate for that if he practices it, then simply observes his results and make the necessary visual corrections. In short, he may have to see things as slightly triangulated in a particular orientation in order to correct for the perception error and make cuts that are actually parallel. This would account for the perfectly uniform tapers you're seeing in his work, because to him the cuts may look perfectly parallel as he does them. It might just be that the curve of the blade is visually throwing him off.