My teenager has started stuttering...

My teenager has started stuttering... My teenager (17m) has suddenly started stuttering in the middle of a sentence. But I'm not sure if it's a speech thing because his bottom lip/jaw starts quivering. He's never had any issues with speech or anything, he's always been healthy and no issues with health ever, which is why this is scaring me. I know it's not drugs and he hasn't had any head injuries. I've heard anxiety can cause it which I can understand, but why would his mouth quiver like that? I'm taking him to a doctor but I'm sure they'll brush it off and if they do, how do I get him a CAT scan or what all needs to be looked at? I feel so clueless