This happens to lots of people who aren't accustomed to or adept at speaking publicly. It's a vocal hitch. I used to do IT trainings and then post my own recordings and never realized how many times per hour I used the interjection "um," but hearing it back allowed me to work on being more attentive to my own diction. Seems like a peculiar thing to razz him about. I'd be giving him the business about his batting average, personally.