I think It's a happy ending cause he flies the plane for the second time at the end to summon him. The picture of shiruo with his parents could also depict that the mc succeeded in what he went back for since he's happily smiling along with his parents. Also we were only shown a picture of shiruo with his grandpa in the beginning but the picture changed to a picture with his parents indicating shirou's parents might still be alive and that he grew up in a loving family like the mc wanted.
(」゜ロ゜)」📖