Sometimes a diaper rash can be caused by the diaper. Assuming he's in what we used to call 'paper diapers', you could try changing brands.
My DS used to get terrible rashes when we visited our family in another state. Then we'd go home and it cleared up. I used cloth diapers at home. Having said that, the 'paper diapers' 40 years ago were saturated with perfume, and that could have caused the problem. Also using the local water in his formula could have caused some intestinal issues that made his waste more caustic.
Desitin ointment worked well on the rashes. A warm wash with salty water might help, as well. That's cheap and won't do him any harm. I wouldn't use any steroids without a prescription.
My daughter was allergic to milk...once we found that out, her rashes cleared up quickly.