My Son (age 8) Has A Rash On His Arms,

My son (age 8) has a rash on his arms, thighs, and face. He had a light rash several days ago that faded after his shower we thought was from being in the sun all day. It faded. The rash tonight looks much worse. It was not there this morning. He says only one tightbis slightly itchy. No fever.