Child Having Sensitive Skin Has Got Itchy Rashes Where Skin Is Dry. How Long Will It Remain?

My 4 year old son has a rash that covers most of his body, although I know from previous experience that he has sensitive skin to soaps &c this episode seems somewhat different. The rash does not appear severe but its continuance and area concern me. It mostly appears as chilled skin (like goose bumps) but dry, almost like sandpaper. It ranges from being very visible to appearing almost completely gone. It has come across as extremely itchy and I have done several things to combat these side effects but it still persists. The only time welts or irritated blotches appear are when he scratches and only for a short time frame, while the rest fluctuates but always remains. Along with the dry skin he has complained on occasion of light and sound sensitivity. Originally I thought it was an alergic reaction to food or soap and cut out everything "new" I could think of. Though today it seems to have lessen in degrees, I am concerned about how long it has remained and am seriously considering taking him to emergency, as today will be the fifth day.