The relationship between your son and your nephew's son is that of first cousins once removed.
Let's break it down:
Your son and your nephew are first cousins because they are children of siblings.
Your nephew's son is one generation younger than your nephew, so he is one generation removed from your nephew. That's why he is referred to as your nephew's "son."
Putting it together, your son and your nephew's son are first cousins once removed. They share a common ancestor (you, in this case), but they are from different generations: your son is from your generation, and your nephew's son is from the next generation.