He's decided to be quiet until he was older, time flew by, by the time he realized he was old enough to talk without being questioned he had the communication issues. Not to mention he would have to imitate a 6-yr old way to speak. Six years of basically talking in your head will really fuck up your communication skills especially when no one else talk with you. Same thing happened to Ryouma from Picked by the Gods. He has nothing to compare his power too. He isn't aware of how much power he has. No stats, no one exclaiming why he's so powerful. Nothing. As far as he knows everyone can do that type of stuff. He has no idea he's a prince. I haven't reread the story but he IIRC he decided to open a coffee shop early on and is sticking to his goal.