What Causes Brown Spots Around Groin?

My 16 year old son has some brown spots around his groin area, they are located where his cup for baseball would sit. They look like age spots or a small birth mark. They are the size of a dime or smaller. It started with only a couple but now over a months time there is about 15. They are not raised, scaley or do they itch and are not irriating at all.