Hi My 15 Year Old Son Started Getting A Rash

Hi my 15 year old son started getting a rash that looked like hives... thighs, inner arms, stomach... we treated with calamine lotion to reduce itch, bathed in pinetarsol and took antihistamine. The redness swelling and irritation have reduced but the rash has spread further and is now turning into little pimple like lumps, with hard whitish heads...