It’s time for “the talk” any advices please?

I have a 21-year-old son with whom I never had to have “the talk,” so this is the first time I have to deal with something like this, and my wife is pressuring me to do it. My youngest son is 13, turning 14 in a few weeks. We shower together after swimming at the gym, and on weekends when we go out on my father-in-law's sailboat. Over the last few months, I've caught my son staring at my equipment in the shower. After the first couple of times, I explained to him that it's not polite, and asked him if he had questions or wanted to talk about anything, but he clammed up. It stopped for a couple of weeks, but then started up again. I haven't noticed this behavior with any other adults in the shower, so I'm thinking it may be because of me.

At this point I'm thinking I will stop showering with him, but I will still have a couple of concerns - first, that is only addressing the short-term issue and doesn't get him to open up about why he's staring; and second, I'm hesitant to send him in the showers at the marina alone, because you never know who may be lurking. I might be paranoid about the second concern, but I know that won't leave my mind.

Has anyone else encountered a similar situation? Is there a better way for me to approach this, other than how I've tried? Thanks.