Why won't my five year old son who is kindergarten color in the lines?

My son started kindergarten this year in August. He has been coloring with me since he was two years old. But, obviously he was young, and I expected him not to color in the lines and scribble. Now that he is almost six years old, he still just scribbles and doesn't stay in the lines. What is wrong with him? Why won't he color in the lines? If you are a parent, have you ever heard of a child not coloring in the lines at this age?