If I may, the MC studied abroad and fell in love with ML, a good classmate whom eventually became his fwb. When it was about time for MC to go back home since his period of studying was almost done, there was one question coming from ML which gave off the impression that they should put an end to their relationship.
MC then decided to do so instead of hoping for his affections, however when MC arrived back in his homeland, not only did he changed from being a beta to omega, he was also pregnant.
Since ML lives so far away from MC and seems to have zero intention in pursuing an actual relationship together, MC then decided to raise the child on his own without telling the ML.
Hence, in my opinion, it was not that he purposely hide the pregnancy from ML, there was just no chance of him to tell ML the truth given the circumstances of the way their relationship ended. Like why would you try to reach out to someone who cuts you off and wants nothing to do with you. I wouldn't want to be someone who blames MC's decision who prioritized himself in the moment because of the hurt he was/is suffering from. But ofc, this is just my two cents :)
Also, ML is not a red flag per say at the time of their fwb situationship then, he was just someone who felt no attachment to what he shared with MC since he expected everything to be merely sexual after all, and surely they both equally understand what it means to be fwb. Obly, he is a definite form of red flag now after his re-encounter with MC.
i really hate the, “i have a baby but not going to tell the father it’s his” troupe. so before i read it is it like that?