He won’t get it, because like a heck of a lot of people, he doesn’t understand systematics at all. We will continue to try, though. Here’s a simple introduction to a basic concept in cladistic taxonomy:
One of the central tenets of modern taxonomy is that every group has to include, by definition, all of the groups that evolve from it. So rats did not stop being mammals when the rodent group branched off the evolutionary tree. Every branch on the tree of life is considered to be a member of all its parent branches.
This means, for example, there can be no definition of fish that does not include everything that evolved from fish. Following this logic you could argue that as amphibians evolved from fish, amphibians are fish. Mammals evolved from animals that evolved from amphibians, so mammals are fish. We are fish. While every biologist knows this conundrum, and that there is no biological definition for what most people consider “fish”, they decide not to worry about it because it’s helpful to think about living swimming “fish” as a group. Taxonomy is useful and makes a lot of sense, until it doesn’t.
There are other ways to classify organisms — we could do it by what color they are, or what they eat — but the one method that works coherently is to group them by line of descent.