Here's how it ends (in my mind)

The mangaka sadly died so we'll never get the official ending, but this is how I imagine the ending:
Narita/Orita refuses to give up on Natsuno, but realizes that an aggressive approach isn't going to work. Instead he offers to support Natsuno in whatever way he can, and keeps trying until finally he regains Natsuno's trust. Natsuno then breaks down and tells him everything. Narita comes up with a plan to help Natsuno escape his brother, meanwhile the older brother (I don't know his first name, I don't think they said it) is carefully scheming with the other doctor to find a way to get rid of Narita. Given the type of person he is, it's something extremely fucked up and illegal.

However, he suddenly loses his mind after Narita threatens to send his boss and coworkers a video of him raping Natsuno unless he leaves Natsuno alone. In the video, to make sure no one mistakes it as being consensual, Natsuno talks about how his brother started abusing him as a young child, which the brother confirms doing since he thinks no one is listening.

Natsuno and Narita then go into hiding together for a while (the older brother seems like the "if I can't have him, no one can" type, so they go into hiding for Natsuno's safety), and the older brother is left to deal with his rage and panic alone. He takes it out on the other doctor, who actually enjoys it. Eventually he's forced to accept that Natsuno is no longer a helpless child and will not allow himself to be controlled anymore. He comes to rely on the other doctor, and they're able to fulfill each other's twisted, toxic needs and desires. The ideal ending would include the older brother getting justice for his evil deeds, but rapists and abusers never seem to suffer consequences in yaoi for some fucked up reason. So I think the second-best ending is that the brother finds another relationship (one in which his new partner actually wants to be subjected to whatever the seme does), since that will ensure he'll truly move on.

Natsuno regains his self-respect and confidence, and eventually is able to put the trauma of his past behind him. While they were in hiding together, they became much closer (I picture Narita sweetly cuddling Natsuno when he's too frightened to sleep alone), but Narita held himself back, because he didn't want Natsuno to feel like he only helped him to get his body. Narita doesn't realize it, but Natsuno already fell in love with him during that time. After it's all over, they begin a wonderful loving relationship with mutual respect and trust, and lots of hot sex and consensual BDSM. When Narita gives him a ring for his birthday the next year, he puts it on his *finger*.

The end. ╮(￣▽￣)╭