Judging by the few panels we see of his past relationships, it looks like he was made to be the bottom in a few relationships, though whether he wanted to is dubious by the way it looks to me.
He seems to have taken the lead with Taejun though. And his past behavior with his last school partner and some of the implications in previous chapters allude to him having a dominant personality that seems to be undermined by the whole herbivore=submissive vs carnivore=dominant social angle.
Can't edit, but it occurs to me he either has wanted to or has topped in the past (we see discussion with his friend about scaring off previous partners - though that could he him wanting to be the top or them taking him for granted/wanting him as a trophy/treating him like shit because he's an herbivore and him putting them in their place).
I think he told Taejun that partially because it's his preference, partially because it's a test to see how he'll react ("herbivore top?!" Or something) and also maybe because he recognized Taejun.

Haven’t kept up with recent chapters but just curious about this, ml was bottom in his past relationships or was he forced into submission as the top?