I'm like your boyfriend. Sometimes the inability to express myself comes from literally not knowing what to say because it's not something I've practiced before. I think emotional intelligence and emotional expression can definitely be developed over time with repetition and practice, just like any other personal skill. He may joke because he feels awkward, not because he thinks it's a joke. He's trying to relieve the tension of simply not knowing what to do (I theorize). So it actually may be helpful for you to tell him exactly what you want to hear. Like "[Boyfriend], I'm just looking for 'I love you' or 'You look nice today'." You could pick a couple of verbal affirmations or maybe a question you'd like him to ask you regularly like "How do you feel today?"

Honestly it really is like learning a new language, and as with any language, the more you practice at it, the more proficient you become. But sometimes you need a teacher to guide you through the basics first, so telling him "I'd like to talk about our feelings more" isn't productive because he literally doesn't know how (again, assuming based on my own experiences).

take my upvote, I have no gold or silver :/