Generally, a white patch on skin isn't cause for concern. Skin discoloration can be triggered by a number of causes, including:

• Eczema. Sometimes a small area of skin becomes irritated or develops mild eczema, which results in a patch of skin that's lighter in pigment than is the skin around it (pityriasis alba). This is common in children who have allergies, asthma or eczema. Your son's doctor may recommend applying moisturizer to the affected skin and avoiding harsh or drying soaps. Your doctor might also prescribe a mild medicated cream, such as hydrocortisone.
• Superficial yeast infection. Another possible cause of a white patch on skin is a superficial yeast infection (tinea versicolor). This condition may cause the skin to look either lighter or darker than usual. In this case, treatment may include antifungal creams or shampoos.
• Other skin diseases, including psoriasis and vitiligo, also may cause a light or white patch or other discoloration on skin. Most often other lesions typical for these conditions are present elsewhere on examination. Treatment of psoriasis focuses on controlling the length and severity of flare-ups. Treatment of vitiligo can be challenging and therefore a correct diagnosis is required and prescription treatments can be considered.

Consult a dermatologist for an evaluation if the white patch on your son's arm continues to enlarge or if additional white patches appear in other places.