Hello: Your child has obviously gone through an episode of possibly infectious colitis/enteritis. These stools are very acid and can more often than not irritate the diaper area. He is now dealing with the aftermath of the infection, which is scalded, irritated skin. You should treat this as you would any diaper rash. It is always a good idea to protect the skin under the diaper with a heavy coat of protective ointments of creams with every diaper change, specially during an episode of diarrhea. It is important to check with your pediatrician in case he has a secondary skin infection, by yeast or bacteria, in which case your baby will need a prescription topical cream.