15 year old son's speech

I have a bright son but something of concern is his age and his speech.

He's been like this from time he was younger but I realize by now he should have "outgrown" it.

In a few words, he doesn't speak confidently.

He tends to mumble like he doesn't want to be heard; he uses wrong words, wrong tense, wrong context or mispronounces a word.

When he gets excited and tries to speak fast and louder is when he seems to trip up the most. Any ideas or suggestions?

Thanks,
JB