My fiancé, I've noticed, has dark gray scabs on the inside of his bellybutton. He says they are sore when touched. I've tried to get him to use peroxide on it. He says he cleans it when in the shower so its not dirt. I'm just worried what this could be.

Perhaps he does not clean it good enough. It needs to be cleaned, and perhaps use some ointment.

I heard of this before, make sure he drys it every time it gets wet..