I think I've quite a similar stutter to your boyfriend, in that some people in passing wouldn't maybe realise that I had one. I'd be someone who would bring it up though, I told my fiancé even before our first date. If he hasn't brought it up I wouldn't bring it up. The reason I say that is because even though I'm fully aware I stutter, I live my life in a way that isn't defined by it (as much as possible) and I've had a few instances were someone has brought it up and it's felt so jarring to know that other people are very aware I stutter even though I don't notice myself doing it as much anymore. It can be quite disheartening whenever you don't want to be defined by it. Saying that, I completely get why you want him to open up because you want him to know you 100% support him and it doesn't matter (which is so lovely). I'm sure that he will open up to you at some point, i just can't imagine a way to bring it up that would be subtle enough especially because he hasn't brought it up yet - as I think a lot of people do bring it up right away. As long as you are doing the right things such as not interrupting or looking irritated when it happens or finishing his sentences then he'll know that you support him and are a safe person to be around. I hope this helps but please please don't take my advice as law, if there's a moment where you think that you could bring it up then you should, after all you know him a lot better than I do! I hope all goes well and I think it's amazing to see someone who doesn't stutter come to this sub to get clarity x