Diaper rash

I'm a FTM. My son is 5 weeks old and this is the second time he's had a diaper rash. The first time it wasn't too noticeable really, and it went away within two days using ointment. This time it's still not too bad but looks a little bit bigger if that makes sense than last time. Is there something I'm doing wrong that he gets this? How can I prevent it? Should he go to the doctor?