Why Are The Itchy Rashes On The Skin Formed?

My son has had a rash that is itchy. It began on the outer thigh of the right leg, and he said has now spread to the left leg. He said it the worse when he wakes up, and seems to calm a bit over the daytime. The bumps are hip to know, only outer thigh, not raised and skin colored (not red). They don't weep, puss, or have any discharge at all. They began to go away for a day or two when he went to his dads, but seem to come right back when he came home (coincidence?).