What Causes Red Itchy Rashes In A Child?

Hello! I have 8 year old son, who have rash - redness and itchiness on the chest, arm pits, arms, legs, neck (front). Also he have already skin thickened on the front part of the neck and on the back side of his knee, yesterday he has a slightly swallen ancles, but they look better today. On the side he have not very high fever - around 100 F. I guess because of the fever he doesn t feel very well. He didn t take any medications at the moment the rash started, normally he don t have any food allergies that I am aware about.