Okay people, let's see what we can see.
The vampire guy was "constantly holding [himself] back" and "today was [supposedly] the final day" for him. Not to mention that he tried to shoo them away, or rather, "get [the] blood away." He also mentioned the incoming "rush," which I presume as the thing he's trying to hold back.
So here's my educated guess. The guy is defo a vampire, and he's abstaining from drinking blood. So far, we've never learned of what will happen if a vampire stops drinking blood altogether, but something is happening. Perhaps they will return to be a human, who knows? What I'm sure of is that the longer they abstain themselves from blood, the stronger the "rush." This brings me to my second point; an incredible thirst for blood. Perhaps that is the very reason why this nameless vampire guy abstains himself from drinking blood. Maybe, the first days of his vampire hood were fun and games, but he slowly turned into a monster as time went. I mean, that's kinda a trope already. So, in order to turn himself back to be a human, he stops drinking blood altogether. However, what gives him the idea of how to turn back, or rather, who? The detective lady said that, and I quote "Rumors sure are a mystery [to her]. It's like saying smoke can actually exist without fire." However, we know that when there's smoke, there's fire. My guess is that MC was deliberately led to go to the school at this particular time. Remember, Kou and Akira were invited by Mahiru, and Mahiru is associated with another vampire (go check chapter 23 and chapter 25) which I guessed as Kiku that was mentioned in chapter 22. My last point is; there is someone who plays in the background, pulling strings and making rumors.