Thanks for using HCM, I understand your concern and will try to give you my medical advice, obviously your son have some intestinal disease, the rash around his penis and anus is a typical sign of osmotic diarrhea that occur for acid stool and the cause is probably some indigestion, the most important thing right now is avoid a dehydration you need to provide electrolytes and water and take care about his diet( soft and healthy)some mint and chamomile tea can help too. I hope that my advice has been helpful, take care