Hi My 7 Year Old Son Has A Red Rash

Hi my 7 year old son has a red rash on his face, his arms, his legs and his hands. It appears to come and go. He had yesterday when he got home from school. It appeared to go away over night and did not shows signs while at school today. Now this evening it has reappeared on his arms, hands, and cheeks, He was scratching at his cheeks. We applied Benadryl gel.