I want to be more angry, but honestly as far as "dense MC" actions go, this one is completely understandable. He doesn't want to ruin what he has now. I bet half the people complaining about him chickening out are also screaming a Yuu for shooting his shot with Hikari at the worse possible time.
What I'm more pissed about is she says she heard him, they both know he means more than friends, and they're letting things go at just that.
Yeah, this is a drop. if the author doesn't want them to be in a relationship, don't write an entire arc to get them to confess and then just screw it up