Did you watch the trailer? Because personally I thought he sounded far more natural and older there than he did in Ender's Game. Which makes sense, seeing as how he filmed EG over three years ago when he was in the middle of puberty.
Look, I get that you don't like Asa for the role. That's totally fine. You're more than entitled to have your own opinion. But I'm kinda surprised by how vehemently aggressive you are that he isn't right for the role at all. While he doesn't seem to be the exact favorite for most people here, others generally at least recognize that he has potential in the role. There are things worth being a bit apprehensive about, like whether or not Asa can pull off the quips and general comedy of the role, but it's certainly possible that he can do it.