Well to be fair, he's kind of a plain guy with seemingly low self confidence, and she's a very pretty former idol beloved by many. Even if it seems like it should be obvious that she's interested in him, a guy like him is going to doubt that it could be a possibility. Although, even if he did immediately shoot the thought down, he did for a moment consider the scenario of him being a love interest.
I feel like this would be so much better if she just confessed first chapter and they started dating. Because I'm really not remotely sure on what this "progression" is meant to be.