I've heard that kneeding is a comfort thing for cats. They kneed their mothers breasts when they are babies to help bring out the milk, so often they associate the kneeding with the comfort of being with their momma. Maybe now George is just more comfortable around you and he is knows that you are his mommy. Tibby likes to kneed on me all the time, I think it's so cute she purrs so loud and has that "happy kitty" look on her face.