it's probably a good thing he gave him away. He was grieving and not in a fit state to raise him, this decision was a pretty important safeguarding decision (which encompasses helping a child achieve their full potential not just risk protection). Although you can't blame someone for their mental illness etc, having a child supporting an adult with mental health struggles will take a major toll. I see it as the best of all worlds (relatives having the family they want)
there is no drive to reveal this to mari with the way the story is written. It stops being told from the perspective of Mari pretty immediately, and there's no hint at any questions or feelings his end. If the mangaka were to choose to have this emotional development (shock, falling out, coming together, whatever) there would have to be a build up. This kind of emotional exploration doesn't suit the style of story or writing — it's not the focus — I'm not saying it shouldn't happen but there'd need to be several more chapters to create the interest and tension required to have the reveal be satisfying or exiting to a reader. As it stands, at least for me, it'd just be "why?" There's no cause or trigger to tell him and the story seems to be finished so I'm content with him not knowing (this is in part due to him not being very fleshed out as a character in this conclusion — if the mangaka did choose to have this arc, there would be more emotional involvement and therefore more impactful. But not every story has to be like that. I find this story fine — not 5 stars, but fine.
A first love that costs 860,000 yen