Hi amal7072,

I understand completely because my daughter is in 3rd grade and often we have assignments to create sentences or stories using the weeks spelling list.  Sometimes, she is so creative and writes so neat and other times, her sentences are short and written sloppy.  At these times, I will have her re-write her sentences and to add more descriptive words or add to the sentences, even if it takes many attempts to create a satisfying result.

What I have found, it depends on her "state-of-mind".  If she is alert, fresh and in a quiet place, she does well.  If she is distracted by her little brother or others, TV or if the weather outside is "just too pretty to work", her work is quick and sloppy.  If it's the weather, I let her go out awhile and enjoy the sunshine and then have her focus on her studies, as she will tell you "you just can't think when it is soooooo pretty outside".  I agree.  

I provide her with lots of paper and several sharpened pencils and give her a quiet spot away from distractions and she writes well.  This may help in you situation, but I wouldn't worry about a little inconsistency in your son's work as I think some is to be expected at their age.  Just keep encouraging their work and as my daughter has learned-it's better to do it correct the first time and avoid the extra 30 + minutes re-doing it.  

I know it can be frustrating for you, but be patient, it will improve.

Best Wishes!