@soulserena I'm afraid you've made it taboo, which makes it fascinating. You've literally crafted a scenario where you're going to get
more potty talk, not less.
The best thing to do with these things is ignore them, and just occasionally correct in a very low-key way if your really have to ("No Timmy, we don't talk about poo or wee while we're eating", or "Nana doesn't like talking about poo, so only do it with other people, please").
If you don't turn it into a big issue then it quickly loses its appeal, but if you make it into a whole
thing then he won't stop talking about it for months or years.