Advice from mums

My nephew has developed a really bad nappy rash. To the point where my SIL is worried about blistering. She uses daptarin (?) and is worried that he has developed some kind of immunity (is this possible?). Anyone have any advice?

We are taking him to the doctor tomorrow, and I am planning on spending some nappy free time tomorrow. But I don't know anything about this - so any advice interweb/mums?