Stop the lesson H4L. There is no reason to keep going with him underminging you in the background.
He's teaching his son to be disrespectful and you are encouraging the lesson. It's your job to stop that! Cheerfully tell S you'll do this later and STOP.
Good for you not taking the bait--he wanted a fight, obviously, and you didn't give it to him. Let him fight with someone else--NOT YOU.
