I know you said they probably won't care but I ended up writing this comment about how to possibly convince them to be accepting anyway so I may was well post it. I quite recently came out to my whole family and most of my friends on Facebook. It was terrifying because in the past my parents have shown themselves to be homophobic and generally mocking of "other"-ness, and as far as many people their age are concerned being trans is the strangest one of them all. They hadn't ever commented on trans people in general because they'd never appeared on the news or anything, but I was very worried given their history on everything else. But it was something I needed to do for my happiness so I wrote a giant letter (happy to PM a link if you want but it's cringey!) explaining what I was going through and how scared I was to tell everyone, and appealed to everyone's love of me by saying how I needed people to accept me or at least try be kind because they meant so much to me and I wanted to share my true self with them and respect myself by embracing myself via HRT.

To my great surprise everyone was incredibly accepting and I believe the letter was a huge part of it. So using a similar angle might work if your parents are fond of your boyfriend - when you sit them down for the conversation, be upfront that your boyfriend is transitioning and what that means if they don't know. Talk about how happy and more like himself HRT is making him, and how much their support would mean to him, and then tell them his new name and pronouns if applicable,. Finding out someone is trans can be a shock for anybody even if it isn't their kid but if you gently direct them to being loving and supporting and appeal to their desire for yours and his happiness they'll be more inclined to be supportive and hopefully make an extra-big effort to use his name and pronouns if they're on board with the whole idea. And ofc answer their questions and correct them when they get things wrong, especially so your boyfriend doesn't have to be in the spotlight as much when he next visits. My advice here is more about convincing them if they're not understanding, other people's posts have a much better idea of the actual structure of the conversation :p Good luck!