Stretching the writing muscles

Here's an exercise I pulled out of a writer's book I'm looking at (The Writer's Workshop by Gregory Roper). I'm to describe someone. This was supposed to be a page (6.6 on the Word counter (double spaced)). Critical comments are required.

Cai
Cai is a speedy ghost—-his pale form and blond hair streak past you jumping, running, climbing on his way to something he’s focused. The freckles smeared across his nose and under his grey eyes lead you to think he’s all summer’s child, yet he was born early in the morn of Christmas Eve. Clouds do roll in when he is frustrated or when he commiserates in empathy with your pain or misery. Then, like the blink of a firefly, as if the painful incident never happened, he’s off to growl with his dinosaurs or storm his castle.
Like most boys he’s full of contrasts: his appetite balanced swinging from glutton to faster almost everyday. Tears drop easily, too easily at times, but a few moments later the emotion disappears like a stone flung into a lake. He doesn’t like the sight (nor even the mention) of blood, but cartoon violence doesn’t faze him. The world is apprehended through touch and the pitch of his little boy’s voice in his questions. He hasn’t mastered jokes yet, but he’s improving. I know the boy Cai, I wonder what the man will be like.