He wants you to buy it. You are amazed. You've never bought that kind of cereal, and he's never eaten it. Probably from television. Toddlers don't seem to pay much attention to television, but they are often aware of what is happening on the screen.
Commercials are very appealing because of the action and the noise.
The food most often advertised during children's television programs is cereal. Some of these cereals are nutritious, others are not. They are more like candy than cereal. Look for the list of ingredients on the cereal box. They are listed in order of amounts.