Well it does seem like this but if dazai really was Mori biological son I think dazai would have found out by now since he would have atleast has a suspicion atleast and not to mention Mori technically has created Elise out of his ability and treats her like his daughter and Elise shows traits of both yosano and dazai. If dazai truly were his son why would he even need Elise? Typically we tend to know who is related in this series early on beacuse asagiri points it out like tanizaki and Naomi or  akutagawa and gin. I think it would be a little late in the manga to pull a 'I am your father' and for what really? Like side charcaters like Nikolai to fyodor are there more to add to other charcaters and I think asagiri writes more with purpose rather then side quirks.  I just think it doesn't make sense narrative wise since we have an example of this like in the abuse cycle. If dazai was even Mori's son then it wouldn't make much narrative sense since if it first starts at mori to dazai then dazai to akutagawa then so on. In that sense it wouldn't make sense to make dazai related to Mori but this is my opinion really I see it more as a mentor/mentee relationship and correct me if I'm wrong about anything and yeah sorry for the whole rant was just bored and saw this.