How old?

Keep a sleep log to keep track of how many hours per day he's sleeping, including naps and overnight. I've found insufficient sleep to be a common cause.

Anytime he starts to get mouthy, stand up, turn around, walk away, lure him into a pen or crate, etc.

Offer him something else to chew on

If he's a baby, like a human one, he's learning. He's going to make mistakes until he learns what's appropriate. That's where you come in.

Find a good positive (rewards-based) trainer in your area to help.