How can I determine if a student would enjoy or has an aptitude for CS?
You shouldn't "test" somebody to check whether they're interested in something. You should give them some stuff you think they might enjoy, and you let them decide for themselves.

There are a ton of programming languages designed for novices, including:


Processing. This is my personal favorite. Shameless self-promotion: I've written a series of tutorials for people exactly like your nephew, available at HappyCoding.io.
Scratch. This is geared towards younger kids, but might be a good "game-like" introduction.
Game Maker. If your nephew is interested in game development, this might be worth checking out. I suggest not limiting him to game dev though.


See also:


What would be a good first choice to teach game programming to beginners?
Using Processing as an entry point in an Introductory Course
Order to Teach Topics in an Intro Programming Class