I’m going through this right now. He has diarrhea and bad diaper rash from pooping so much. You should call your dr to test his poop since you were just out of the country. For the rash we are bathing him 3 times a day in his old baby tub with warm water and 2 tablespoons of baking soda. Then you have to pat (don’t rub!) his booty dry. After the 3 baths we put lotrimin on his butt. It’s for athletes foot. All the other diaper changes we slather on the red diaper rash cream. Anything with 40% zinc oxide. It’s gotten a lot better in the last few days even though he’s still pooping so much. The open sores are closing up
Each diaper is made with different materials, many companies put additives or fragrances that help with color, absorption, and odor. It very well could be the diaper causing frequent rashes even if you're changing them often enough. (I had a baby who had frequent rashes with all except 1 brand) I use joboba oil or coconut oil with aquaphor on top as 2 of my babies had reactions to products containing zinc oxide. Aquaphor heals like magic. I really hope it's just a stomach bug that goes away soon.