because after the timeskip, we can see that he's visibly grown taller and more mascular + there's also the fact that they quite literally censored his grown up appearance in the afterword, so it's clear that the author gave the little high school vampire kid a complete glow up after military. and also, it's kind of just an unspoken understanding in yaoi fandoms that the physically bigger guy is the top (it's a stereotype and it shouldn't be that way, but oh well)
Whats has being taller and masculinity to do with being a top
i said its a stereotype and it shouldn't be that way dawg but it unfortunately is and that's why the fandom unanimously decided that he'd be top
how does everyone know the vampire is the top? not complaining. just genuinely asking