Sounds exactly like my DS at that age. I'd keep an eye on the writing, but I wouldn't be alarmed. My son constantly got in trouble for not finishing his work and for his slow writing. I think a lot of the problem was because he found the writing prompts too simplistic. For example, he once responded to a writing prompt with "anyone who read this book should be able to answer this question...it requires no thought whatsoever." His teacher wasn't pleased but I could understand why he didn't want to write several paragraphs on Judy Moody when he was reading Harry Potter at home.
By the way, he is a wonderful writer now. And while he still prefers to formulate a near perfect product in his mind first, he has learned how to put his thoughts down more quickly and rely on the the editing process to flesh them out.
