Hello, My Son Is 6 Years Old And His Weight

Hello, my son is 6 years old and his weight is about 54 pounds. My question is I just noticed him itching his skin on his chest. So I took his shirt off and his skin on his chest, neck, belly and under his armpits are very red and he says it is very itchy. He has never been allergic to anything and he is not sensitive to the skin. I asked him when he started feeling itchy and he said at school is when it started today. What do you think it could be? I'm worried. Thanks, Michelle