Weird skin rash on my boyfriend?

my boy friend has this really weird skin rash between his thighs. its not on his man parts, close to them but only on his thighs, it itches him alot, kinda swells, and feels very dry, its not a bunch of bumps or sploches but is one big red spot. he is in the mariens so he has been looked at by a dr and has been tested for stds and they havent told him anything, im thinking it might be either a yeast infection or a sweat rash. anyone know what it could be?