Is there any other reason for his dry skin? Just make sure his thyroid
is fine. Does he have normal tear production? If you are sure that other things are fine and just a very dry skin all over the body is what troubling him then ask him not to use very hot water for bath. Avoid shower bath as it will dry skin further. After bath ask him to pat dry the skin and apply liquid paraffin
oil all ver body when the moisture is still there. He should use soft cotton clothes to cover even the arms and legs fully. Not to use soaps for the hand wash frequently or unnecessarily. And he also can try urea based ointment like cotaryl after a few hours of bath. In spite of all these measure if he's not fine, then do take him to a dermatologist
who will rule out allergy
or any other reason for dryness and itchy skin. After evaluation he/she will prescribe appropriate medication. All the very best.