Childs Jaw Flutters After Speaking

Could it be he is repeating what he said or the response he received? I noticed my Nephew and my Daughter did that at about that age and sometimes they actually muttered loud enough that I could hear that was what was happening. It's almost as they are repeating it in their heads but their mouths follow a little bit. Watch to see if that makes sense as my thought was it's a developmental/learning stage related to audiation, an example for us is imagining a song in your head after you've heard it. If so, it goes away for the most part but I think we all have a tendency to repeat things we want to stick in our brains.

I liked Tori's suggestion of what this may be. Not being able to be there and observe first-hand like you, I can suggest that there may be other things. There is something else benign called a transient motor tic which a high percentage of children have for a little while and then it goes away.

If your son has recently started or stopped some medications, it may be a temporary side-effect of that.

My nephew silently repeats the last thing he says sometimes...if that is indeed what your son was doing..

Sounds like a nervous tick. Let the doctor look at it but don't take any of the medication she/he offers.
Acupuncture would do a better job of stopping it once and for all.