it's his grandson
That's right but if you reread it he says that he is grandson from his mothers side
http://www.mangago.zone/read-manga/aigan_shounen/an/aigan-shounen-chapter-5.1.html/11/
Haruhito Kyose was the soldier boy's full name.
He says in the scene you link that his grandfather died of Illness. He was talking about Haruhito's dad, Kyose was the family name.
"grandfather on his mother's side" that means Kyose was his mother's maiden name, Haruhito had a sister.
So the boy at the end was a nephew.

I don't understand koyse is alive or what in the and?????