My son had gastro and we got smecta and rehidrate and his stool is slowly getting there but now he has a rash all over his body. ?

Viral infection: The rash is probably from the virus that caused the gastroenteritis. We frequently get sick, have diarrhea and then days later a rash pops up. It all likely comes together just related to the virus that made him sick. It was not likely related to anything you did or gave him.

Viral exanthem(rash): Most cases of gastroenteritis are caused by viruses. Many viruses also cause rash to develop, usually a little later in the illness. As long as no open sores, drainage from rash or pain associated with rash, just monitor. You certainly can us Benadryl as needed for severe itching. Otherwise, give oatmeal baths, keep clean and loosely covered to avoid scratching it. Should fade in a few days!