That's it. He wanted to experience love for the last time before he dies, but he knows that when he's gone, the girl he's dating will be left alone and feeling sad. So instead he decided to have an affair with a married woman who already has a "life" (husband, family, house...) so when he's gone, his absence will mean nothing to her :( Buuuuut Shoko is not married so his plan is not going to work.
In the manga he's at the hospital with his sister and doesn't remember Shoko, so I suppose he has had that surgery.
No, it hasn't ended yet, but everything that has already happened so far in the manga is quite sad. He has a brain tumor and doctors say he will probably die soon. Then he faints, and after being taken to the hospital he has amnesia and has forgotten everything about Shoko... :( Hope it has a happy ending!!
So instead he decided to have an affair with a married woman who already has a "life" (husband, family, house...) so when he's gone, his absence will mean nothing to her :( Buuuuut Shoko is not married so his plan is not going to work.
In the manga he's at the hospital with his sister and doesn't remember Shoko, so I suppose he has had that surgery.