It sounds like you agree that billionaires are overcompensated relative to their contributions to society. If taxes aren't a good way to address this, what is a better solution?
Daytrader, yes. Flight controller, no. Unlike the buffoon with five kids who loses his shirt, your son's losses would presumably be limited to his allowance, at which point he'd learn canning might be more his calling. On the other hand, his chances of succeeding in forex are about as good as anybody else's.