the Prokids Flare boards are designed for kids and itīs great to watch how much fatser they improve on these shapes. I would say that the size decision depends on in which he is sailing. If itīs super light most of the time I would suggest the 72 one. If it ghets windy most of the days I would suggest the smaller one. Roger is absolutely right, I guess, that your brother will grow much soon.
I would tend to the 72, which offers a great performance in lighter winds, still if your bro gets a bit heavier. And in lighter wind conditions you definitely improve most when learning newschool freestyle.