[andymism]

Assuming your cofounder is also involved in the design of the product, the easiest way to show him the rest of the iceberg is to sit down and write a functional spec of the feature together.

As a non-technical founder, he's likely just thinking about the end result of feature X, not the implementation of it, whereas you see the technical details at a much higher resolution.

You don't need to do this every time with him (although it's not a bad idea)--just once or twice will be instructional enough. The goal is for him to understand the amount of thinking and problem solving that goes into feature X before a line of code is even written.