It kinda is his fault. Sure, low self-esteem and all, but with how much they've interacted already, you'd think he would entertain the idea that Kubo might give him some chocolate even if just to tease him.
I would accept it if he thought it was just obligatory or friendship chocolate or something, but to not even suspect Kubo, the only person in this school who consistently notices and interacts with him, is a bit upsetting to me. It would be easier to buy if he didn't have this bunny girl senpai-esque curse of his. Maybe he should wear weird costumes too to get people to notice him.