Wow , I’ve tried some prostate massagers and plugs And only dreamed of leaving it in while with others or in public
That’s just to accessive in my opinion

I think how you bring it up with him is whats going to define the entire conversation - but that conversation definitely needs to happen.

I know you've tried before and it's hit a dead end up sometimes I think being straightforward is really the right path. There are lots of times I try to beat around the bush, subtly bring something up so that the other person still feels like they're in control and not being backed into a corner. But when he's obviously deflecting and refusing to progress the subject then sometimes you have to put the nice girl routine back on the shelf.

Starting it by simply saying, "I want to talk to you about the fact that you're wearing your plug all the time," will tell him exactly what needs to happen. I think its also important to state fairly early that you aren't mad, or upset, or judging. You don't even necessarily want him to stop - you just feel secluded from him and you're worried that gap is getting wider. Make sure, before you get too deep into the conversation, that he knows you simply want to feel like you're a part of this piece of his life (or understand better why he doesn't want you to be).

It could be as simple as embarrassment or that part of what he likes about it is is that its a secret. He could also simply not understand why he likes it so much or maybe it's a lead up to something else that he wants and doesn't know how comfortable he is with letting someone in on that. Just remember that the worst case scenario is not usually reality.

Good luck!