This is so damn cute and sweet. It's also the kind of story that you can view from a trans perspective, though I know it's almost definitely not intended that way.
Early on, I wondered if Masaru was faking his denseness to some extent, but the way he questioned whether Kazu actually enjoyed being called cute made me think that he was being genuine. Then again, that could have been an excuse; maybe the real reason he wasn't calling Kazu cute was because the family vacation gave him time to think about the situation and realize what was really going on, which left him unsure exactly how to interact with Kazu. I'm probably overthinking it, though.