They are only 13-14 so it makes sense Hajime is more into foolery with boys than romantic stuff. Although, given how intrusive Mitsuishi is in Hajime's life, it should be obvious he reciprocates her feelings. I mean, otherwise her behaviour would be just suffocating. She is even aware he is different with "female-friends" than he is with her. Hajime may not believe he is a good fit for Mitsuishi, but I don't think he would reject her if she just asked him to go out with her (officially). She is free to go and do it. In fact, it might actually make Hajime be more reserved around other girls (or femboys XD).
By the way, was Fuyuno actually going to show him her boobs, just because he caught her by her words?