What Can Cause Skin Rash In Kids?

Hello, ealier on today my little brother came home from school with an itchy rash around his collar area. About half an hour ago or so he came dowstairs and his eyes were swollen (puffy) and the rash had spread all over his body. As well as raised bumps on some areas of his skin similar to a nettle rash. However he says he has not touched any plants or digested anything he doesn't usually eat. He doesn't have any other allergies apart from Hayfever- that any of us know of. Any advice about what we should do?