It was Kenny's height that had led Thorongil to move towards him -- finding and protecting Frodo, Sam, and Bilbo was as important, if not more so, than getting safely into the space station. It is only when the knife is pulled that Thorongil realizes his mistake.

In the zero-gravity, he cannot move away fast enough -- he recoils, twists, and moves far enough for the knife to hit nothing vital, but he is still injured, and Thorongil cries out in pain, cursing his luck and his judgment and those who mastermind this game in the first place. He gasps in a breath, taking a second to clear his head from the unexpected blow.

If it is not a hobbit -- and it is reasonable to assume it is not -- then it must be a child. That limits Thorongil's options; even in self-defense, hurting whoever this was would be wrong.

Therefore, he opts to reach out, grab Kenny by the front of the spacesuit (none too gently, but it won't harm the boy, and Thorongil would be lying if he said he didn't want to frighten him a little) and, bracing his feet against the anchored base of the supply-cube, flings Kenny in the direction of the space station. It would send the boy towards relative safety, get him and his little knife away from Thorongil, and let Thorongil get the nearest supplies without further interruption.