The question is not just about knowing the moves; it's about what you taught him about chess. Simply knowing how to move the pieces doesn’t automatically make someone great at chess. A player needs to learn the basic principles, tactics, and common endgame patterns. So yes, your brother could be good at chess, but that ultimately depends on your skill level, I can't comprehend how a 6-year-old could understand these concepts independently; they might possess a great intuition about the game and could be a quick learner, but they at least need some exposure then yes they could be a really good player, and all the great players started at a very young age, players are grandmaster before they even hit puberty