make it fun... get him to write out things he likes to talk about, or even anything he likes. start with writing small amounts and then gradually go on to essays... reinforce for everytime he writes. dont tell him about how badly he has writtten, wrong spellings, etc. just reinforce him for writing. then gradually help him in the other areas like spellings, etc, wherever needed in the same way.

Young writers often struggle because they do not know where to begin and how to organize their thoughts.  Teachers help students learn to use graphic organizers to provide a roadmap for writing.
 
My first recommendation would be to discuss your son's difficulty with his teacher and find out what method and organizers she is using with the children.  Two of the most widely used methods are Four Square and 6-Trait Writing.  Some teachers just use graphic organizers.  If a teacher is not using a specific method or graphic organizers, then many kids will struggle with writing, because they do not have a roadmap to show them the way.  Ask the teacher how you can use these writing tools at home to help your son.
 
If you son's teacher is not using these tools, then arrange to meet with the teacher and the principal or lead teacher to come up with a plan for using them.
 
ReadWriteThink is an excellent website with ideas and graphic organizers for teachers and parents to use with kids.  Here's a link to some of the activities they have to help build literacy skills outside the classroom.  Several of the activities are writing activities:
 
http://www.readwritethink.org/beyondtheclassroom/summer/grades3_5/
 
ReadWriteThink also has several interactive graphic organizers to help kids get their words on paper.  Here's a link to all of their student materials.  On this list, some that might help your son include: Animal Inquiry, Bio-Cube, Book Cover Creator, Character Trading Cards, Compare & Contrast Map ***, Comparison & Contrast Guide ***, Essay Map ***, Story Map ***, Flip Book, Persuasion Map ***, Notetaker, Webbing Tool and Stapleless Book.
 
(*** really good, I've used them)
 
http://www.readwritethink.org/student_mat/index.asp
 
My daughter gets really excited about writing when she can include some of her artwork.  I've encouraged my students who like to draw to illustrate their writing.  Another approach lots of kids like is to create a lap book or pamphlet that includes their writing.  This can be done for any subject or topic.  Here's a link to an example written by a homeschooled student:
 
http://blog.bravewriter.com/2009/10/14/one-moms-lapbook-success/
 
Your son will do well with your support and encouragement.

Hi, In addition to the wonderful information you received from JustAsk member, lravidlearner ...
 
You might also try some of the writing activities on Education.com to make the learning more fun:
http://www.education.com/activity/fourth-grade/writing/
 
Additional resources on Education.com that you may also find helpful...
 
Writing info center
http://www.education.com/topic/academic-subject-writing/
 
Supporting Your Child's Writing info center
http://www.education.com/topic/writing-supporting-your-child/
 
Good luck!

It is very important to keep in mind that in order to be a strong writer you first need to be a strong reader!  Often children are not developmentally ready to begin the writing process - especially if the skills in reading haven't been developed first.  These would include decoding, spelling, oraganization of thoughts, vocabulary, etc.  A strong writer is able to comprehend and visualize while reading.  This is what creates detail, summarization and written expression.  
 
When your child reads a paragraph - stop him and ask him some detailed questions.  Tell him to visualize while he reads and encourage him to see detail.  
 
When writing - start first with some background information.  Ask him some questions and encourage the thought process to begin in his mind.  Use graphic organizers to organize the thoughts and from there put them into writing.  You want to always start with a well written sentence before expanding into a paragraph.
 
Using the computer to read and write is a huge advantage for most students.  By letting young children write and send email, they practice reading, writing and spelling.  Teach your child how to use spell check before sending off messages. Be sure to monitor your child’s ‘pen pals’ – who is your child writing to and receiving mail from? Let your child pick out a few family members, including grandparents and maybe two or three friends.  You will find that by using email regularly, your child becomes very strong in typing (keyboarding) and using the computer.
 
Encourage reading as much as possible.  This is what develops the vocabulary, creativity and also is a model for what writing is all about.  
 
Good Luck!