What happens to Roxas at the end of Kingdom Hearts 3?

first of all he was not killed he was knocked out and he lost all of his memory of the organization and what happened was riku used the dark corridors to take him to his own twillight town and layed him in bed that is the first 7 days in kh2 since in 358/2 you see the video of axel and roxas on the clocktower waiting for xion and it goes back to day 7 to play the game

there is your answer!!!