What Causes Rash On Buttocks?

My husband has a rash on his buttocks. He asked the PCP about it and was prescribed a cortisone cream. It sort of cleared-not entirely. Then it came back with a vengeance. At the next dr visit he told the dr it was back and he gave another prescription for the same cream and was told to use it sparingly. It is just not clearing it up. We have also tried zinc oxide and Vaseline. It seems to me we need to know the cause in order to treat it. I have tried changing laundry detergents and soap without results. The rash is pretty severe and covers a large area and needless to say it is pretty miserable for him. Do u have any idea of what might be the cause?