Neck Fold Rash

Hello, FTM here, my son is almost 2 months old and a bit on the chunky side. I've noticed that due to the fact his neck is being hidden by his giant cheeks it appears he is getting rashes in between his neck folds. I try to make sure that bmilk or formula doesn't make its way in there but I'm not always successful. My pediatrician suggested corn starch but it doesn't seem to have made any progress. Has anyone experienced this before or have suggestions?