The first thing I would suggest is to take your son for a thorough vision exam. This is about the age when many young kids first develop vision changes that can lead to learning problems.

That aside, here is a trick to improve writing: Have him pick out an excerpt from a book he likes, about four sentences of descriptive writing with not a lot of punctuation. Have him write it as fast as he can while you accurately time him. Write the same thing once or twice a day for three or four days. The goal is to improve his speed, and in his situation, his spacing between words. If it improves, he gets a reward -- something inexpensive like an ice cream cone. Then, he picks a new passage and starts the process over.

I would also have your son use the summer to get a lot of practice at computer keyboarding. There are some good children's computer programs to help improve this skill. In the future, teachers may then permit him to type long homework assignments.

By the way, it's not unusual for young guys like this to become more legible once they get accomplished in cursive. I have seen this happen many times. Good luck.