welcome to "healthcare magic forum "
will suggest you to use a mild shampoo as your son is too young and
homoeopathic medicne will help to treat the cause rather than any external factor ,if overlooked it may spread further, consult a homoeopath who will be able to help you to get rid of the dandruff , use to lukewarm water for head bath , clean the scalp
properly with a clean towel ,donoto use that towel for rest of his body and face .
takecare,wish you good health always !