Son has white spots on hands and fingers for about month. now itchy rash on arms legs?

Itchy White Spots: This sounds like something known as dyshidrotic eczema, but without a picture or more description of the rash, it's hard to know. Would check with a dermatologist.

Can't diagnose with: available info. These lesions need to be visualized & a history needs to be obtained in order to make an accurate diagnosis. Sometimes lab testing or biopsy is necessary.