Stop using baby powder, it’s terrible for diaper rash. Do Sitz baths twice per day and let him air dry for a while. Apply a thick cream of choice to create a barrier and protect the skin. Change him often. Give a BRAT style diet, Banana, Rice, Applesauce, Toast, to bind up the poop and avoid fruits that start with P, pear, peach, prunes, they loosen poop. Go see a pediatrician to see exactly what type of rash it is, and maybe give him some Tylenol to help with the pain. Good luck.
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