T.H.
My daughter used to get it like that from fruit too. She loved fruit and ate so much of it. You may have to limit it, especially if it's bad. B is right, that it's about acid. I doubt it's about an allergy, but you should probably mention it to the pediatrician. I loved Beaudraux's Butt Paste. It smells good and it's nice and thick! Sometimes I would apply that and Aquaphor over it if it was really bad. Also, when diaper rash is bad it's also really good to allow him to be without a diaper for a bit. I know that's kind of scary but it does help a ton! If you do it right after diaper change it usually works out and even 10-15 minutes helps.