Protect his feet wherever he paddles. Family adventures across the water sometimes reach rocky shores or gravelly riverbeds. You can worry less about his feet getting scraped when he wants to jump in the water. If it's cold out, neoprene shoes can ensure his toes stay toasty.
A good pair of water shoes drain and ventilate well enough to dry quickly. Too much water weighs his shoe down and makes movement difficult. They should fit snugly but remain flexible enough for his feet to shift around without causing rubbing.