What Can Cause Dry Skin And Itching After Sweating?

my son age 5 years,height 3ft 2', weight 20kg has been suffering dry & rough skin at the face,neck ,back,both arm and legwith itchiness since 3years. There is no family back ground,except sorosis from his dad which found appearing on his both ankle.Currently I am following up with the skin specialist and advice to apply enzyme moisturizing cream and lately they give me Aqueous moisturizing.However when he start to sweat he will feel itchy and start rubbing, the skin will show some white patches.From Raj