Ya know, I would probably take a different route and try to be calm and rational. Here's how I would approach it.
Ask "Honey (or whatever "Bob" would work), is the locker room the only reason that you are so adamant that junior is circed?"
He answers. If he doesn't fill you in, ask "Have you ever had any problems related to being intact?" If he says yes, and says what he had, we can help you understand how it could have happened. Some problems were caused be forcibly retracting at a very young age and with every diaper change. I would try to understand that the world he probably grew up in was mostly filled with circ penises, in fact, his may be the only intact one he has ever seen.
Then, later I would play some bedroom games. I would do things that couldn't be done to a circed penis. After wards, the next day maybe. Say Ya know what I did last night? Junior wouldn't be able to do that if ha was circed. I would also ask if he knew what circ entailed. I would really educate him, gently and lovingly.

Good luck. Total agreement isn't necessary. Just him accepting taht it can always be done but never undone.