One thing I would consider is talking to his teacher about your concern, and learn if the teacher has any specific recommendations for how your son could improve his grade. Here are some guidelines for building a positive relationship with your child's teacher: http://www.education.com/topic/parent-teacher-relationship/
It might be helpful to ask his teacher specific questions such as these:
1. Is the grade based on what he writes (content) or how he writes (legibility)?
2. If its based on legibility, is his writing more legible with certain subjects? Is it more legible if the writing space is larger? Has he recently switched from print to cursive? Is it more legible when he doesn't have to copy from either the board or a book?
3. If its content, you may want to check out the software Kidspiration at www.donjohnston.com to help with organization of his thoughts.