Do you take vacations? It's important to demonstrate what happens when you're not around.
It's likely they won't notice anything at all if he is doing a good job.
That's the other issue: not only the product of programming is not easy to see but consequences of good or bad practices do not become visible before the author is long gone and forgotten.
Sometime doing IT feels like delivering medicine.
There was one time that stands out where everybody expressed gratitude for my hard work and asked if they could help me out. It was when I spent a week running cables and setting up PCs for our incoming batch of interns.
I know that programming is much more challenging than crawling under desks and plugging things in, but none of them ever really saw the programming. All of that crawling, though--I looked like such a hard worker that week.