Don't take it personally. It's normal. My 12 year old stepson has three sounds. The first is a sort of guttural grunt which is his main form of communication. It means yes, no, good, ok etc. Then there is a sort of cry of pain. That means he's just lost or been killed in a game. The final one is a heavy sigh which is used when me or his dad crack a joke.
He is capable of speech as we can, at times, force a conversation out of him by insisting he speaks or he wants to tell us about something he's seen or learned online.
If boys are like girls then he'll become a human being again when he's 18.