5 Year Old Son Has Had An Itchy Radh For

5 year old son has had an itchy radh for a week now. He is sensitive to the sun when exposed for too long. He would usually get a bunch of small really itchy bumps, but now he has a crescent shaped one in his upper left arm and a few smaller clusters of smaller bumps. They are kind of drying msybe from the itching and darkening maybe from itching aswell?. And im not sure what it could be? No change of diet or skin care routines.