Yes he did in the beginning chapters heeseo that's why always cautious of his step brother, since step bro is also second lead and Cha was in relationship with (as author note suggested she hates unfaithful seme's but when uke do that it's alright) hypocrisy is at peak.
Wait so at the begin of the story, you’re telling me he kissed his bro (even if it ain’t biological) ??