Kyle Reese = the real John Conner
Just to be clear: He becomes John Connor after he goes back in time
Again and again he allows the robot to chase Sarah while he feigns injury so he can fall back and attack it in the rear. In the same way he allows skynet to think John Conner is his son so he can feign death and sneak up and attack from the rear
How did Reese lift that car off himself?
Tell me about my son.
He's about my height.
He has your eyes.
What's he like?
you trust him. He's got a strength.
Whatever made him strong and made it possible for him to travel backwards through time also caused him to stop aging. Therefore he is able at a later time to pass as Sarahs full grown son.