I urge you to concentrate on those three colors for the three basic shapes. The colors will lead to further discussions, along with more complex shapes later, and you want a basic group that "works" with any more complicated shapes you may want to add later. (See basic shapes template here.)

Ask your child to pick out all the triangles or all the squares. When he can do that, set up "problems" like "give me the medium triangle" or "give me the large square." This is a great waiting game. It's quiet, the pieces don't dig into your side while sitting in your purse, and who cares if he spends half an hour making rug "pictures" with the shapes?

When he can do this then you can introduce the circle (Venn diagram) and ask him to put the triangles inside the circle and every other shape outside. It's really a variation on sorting, but builds a strong foundation of basic shapes and position, and an absolutely essential basis in logic to prepare him for algebra or other higher math. Even more amazing is how quickly kids that age can compare complex objects to simple shapes (a keyboard is a large rectangle with lots of little squares on it) and will use the shapes very much like the original Colorforms set.

Note--I quickly made adaptations for my oldest as he takes to anything mathematical or geometric like ducks take to water. I added purple hearts, orange hexagons, brown octagons. I added a second circle (on a bigger mat) and asked him to sort medium things into the left circle and red things into the right circle, and where they overlapped there should only be medium red circles right? *G* That one took a while to get but now that he has it he's taken off with all kinds of puzzles and games like Mastermind.

Just let him explore and play with them initially, adding "this is a _____" and "give me the _____" later. Take your time getting to anything past identifying size or shape, and add color after he knows the other two attributes. He'll get it, and have fun to boot!