Bob, I think if you just calmly say, "We both know that's not true, so lets drop it," and walk away, you have gotten your point across to your son that: 1. It's not true. 2. That you won't engage with that type of behavior.
My kids don't remember what me and my X fought about, they remember that we yelled at each other and fought in general. It was quite traumatic for them. With counseling I was able to make changes to me and how I would respond, which helped greatly.
Kudos to your son! Sounds like he is making some really great progress in diving. That is super cool!
I hope you are able to have a good day today!