It is possible that the plaques on your son's hands and arms are due to an allergic reaction. The best treatment would be to try to identify and avoid the allergen, if possible. You can apply a fragrance-free moisturizing cream as often as needed to soothe the skin. Avoid fragranced products and harshly rubbing the skin when drying. If the lesions do not improve after a few days or if other symptoms appear, it would be best to consult a doctor for further evaluation.