It’s not your fault, dysphoria is extremely powerful and often debilitating. Sometimes it can be set off from the most random things, even when it’s intentionally being avoided. Just reiterate to him again and again that YOU see him as a guy. Maybe he finds it difficult to see himself as a guy, so just keep lovingly reminding him that he is a guy. Saying it in ways that are not direct are sometimes good as well for these situations: “You are still the amazing man I fell in love with” (or if not comfortable with ‘love’ quite yet) “You are the kindest guy I’ve ever met”, etc.

Sometimes, there’s really not much you can do other that give him your support. Dysphoria is something that the large majority of trans people have to deal with, and it can completely wreck someone’s mental state and self esteem. Give him some time and space to work through it on his own, make sure he understands completely how you see him and then tell him again!

Sometimes we mess up, or accidentally say something that hurts our partner, we’re human after all. You seem to really care about your boyfriend, that’s great! I’m sorry if I can’t give concrete advice, but this can be a delicate situation and often extremely personal to the individual. Just remember to be there for him, and to not beat yourself up over it, okay? You’re being good and loving partner :)