This is a good questions and your child is not alone.  Writing is a process and it takes lots of practice.  Also, writing shouldn't necessarily be taught in isolation.  Reading and writing go hand in hand.  As you are reading to your child point out how the author used his words.  You can look at word choice, literary elements (such as onomatopoeia, personification, metaphors, etc.), author studies, genre studies, text structures, etc.  Make it fun.  

These two articles are filled with great ideas on how to encourage children to write at home.  I'm sure with more practice, your child will be able to respond more quickly as well as with more thought.

Hope this helps.

Barb K
www.LessonPop.com