Hi, I agree with Michelle to check with the teachers if your child lie belly down a lot in school. Probably you can get the teachers to pay more attention not to let your child so that? I suspect he might be allergic to the cleaning solvent used to clean the floor?
I've read an article, it says it might be food allergy?Some common foods, which cause allergic reactions such as stomach rashes, include wheat, shellfish and peanuts?
You might want to try this bath treatment which works wonder for my kid who often have rashes. After which you can use moisturizer or calamine lotion to ease the itch. Hope he will be well soon.