What Causes Rashes With Black Spots In Groin Area?

My 18 year old son had a rash in his groin area which was spreading in spots,. He went to the dermatologist who said it was a fungal infections and prescribed a foam and a shampoo. It is now about a month later, and he says the rash is back, now ion the front of his right thigh and lower right back. On his thigh it a rash with dark black spots that look like dried up blisters and on hi lower back it looks like welts and they have muscle tension under the and itch a little. He said the black dots are little, and started as some small red bumps about the size of a silver dollar, those turned into the small dark bumps then the rash spread from there.