I am fairly certain that a spatial reasoning question would be non-verbal, with few or no printed words in the question, other than "what comes next?" or a similar phrase. For example, there would be a series of five squares. In the first square, there would be one dot. In the second, two dots, in the third, three dots, and in the fourth, four dots. The fifth box would be empty. The child would have to predict which of the answer boxes would be the next in the series. The answer boxes would contain various numbers of dots, or no dots, and one box would contain five dots, which of course would complete the pattern. Other examples might be a particular colored triangle. First it would be placed in the lower left corner, then the upper left, then the upper right. The answer would be the box showing the triangle in the lower right. It's patterns, predicting sequences, figuring out which shapes would belong in a blank space, etc. So it involves observing, and thinking about which answer option completes the pattern. You can help your son prepare by having him look at the pattern, and not rush. Some kids pick the prettiest or brightest colored answer, or the first answer, or they get flustered.
Spatial reasoning is what allows us to judge simple things, like how big a container we will need for storing the leftover food, and it allows architects, designers and artists to determine scope and perspective and how their plan will fit in a landscape or in the space allotted. It's the ability to look at an empty backyard and say "oh, our swing set will fit there, and we can easily fit the hot tub there", without literally placing them in a spot and moving them around. It's physical and sight judgment. You know how some people can just say "your couch would look awesome over there" and you say "oh it would never fit and if it did it would look terrible there" but then you try it and they were right! And they didn't measure or spend hours dragging the couch all over your room? That's spatial reasoning.
