I would take him to the GP, my ds had bad nappy rash and for some dippy reason i assumed the GP could not help. It went on for too long and he was sore, within a week of seeing GP he was better, and in retrospect feel bad he was uncomfortable for so long but thought it would heal up. It turned out to be thrush.
I kept his nappy of loads and put GP cream on. Always use barrier cream now and the ones that worked help ds are benthanpam, sudocrem and willeda's nappy area calendula cream. hope it gets better know what a worry it is.