It's more appropriate for a much younger child. My DS (at age three) would sometimes smack our family dog on the nose when thought no one was looking. I think he wanted to see how the animal would react.
He was recently diagnosed with a receptive/expressive language impairment (social skills being the dominant issue). Based on this diagnosis, I think, at age three, he did not realize that he was aggravating the animal, and just as worrying, did not realize that an upset animal had the potential to retaliate.
Now, at eight, he is incredibly gentle and kind with animals-- he's also wonderful with young children. I think his ability to "read" things is way behind, and as a preschooler, he needed to understand some things explicitly that most of us just "get." At around 5.5, I really saw empathy/remorse click in and when it comes to those weaker (animals, young children) he is extremely careful and rule bound (i.e. probably much more careful than most same age peers),
My instinct is that your relative's son may need an evaluation- at six, I might begin to work worry about more oppositional behaviors/empathy, but it's still kind of early.
